Function not recognized within scope

I've almost finished writing a program that will detect palindromes from a file and output a new file highlighting the palindromes but I'm stuck on a really dumb error. I'm trying to write a test for one of my methods (TDD) and, for some reason, it's not recognizing the function as within the scope.

I'm calling the isPalindrome(string s) method (declared in PalindromeDetector.h) in my isPalindromeTest() method (declared in PalindromeDetectorTest.h) but, for some reason, it's not recognizing it as within the scoope.

I feel like everything should be working but it just isn't. Any help you can provide would be greatly appreciated. Below is my code:

PalindromeDetector.h

#ifndef PALINDROMEDETECTOR_H_
#define PALINDROMEDETECTOR_H_

#include <iostream>

using namespace std;

class PalindromeDetector {
public:
void detectPalindromes();
bool isPalindrome(string s);
};

#endif /* PALINDROMEDETECTOR_H_ */

PalindromeDetector.cpp

#include "PalindromeDetector.h"
#include "Stack.h"
#include "ArrayQueue.h"
#include <iostream>
#include <fstream>
#include <cassert>
#include <cctype>
#include <string>

using namespace std;

void PalindromeDetector::detectPalindromes() {
    cout << "Enter the name of the file whose palindromes you would like to detect:" << flush;
    string fileName;
    cin >> fileName;
    cout << "Enter the name of the file you would like to write the results to: " << flush;
    string outFileName;
    cin >> outFileName;
    fstream in;
    in.open(fileName.c_str());
    assert(in.is_open());
    ofstream out;
    out.open(outFileName.c_str());
    assert(out.is_open());
    string line;
    while(in.good()){
        getline(in, line);
        line = line.erase(line.length()-1);
        if(line.find_first_not_of(" \t\v\r\n")){
            string blankLine = line + "\n";
            out << blankLine;
        } else if(isPalindrome(line)){
            string palindromeYes = line + " ***\n";
            out << palindromeYes;
        } else {
            string palindromeNo = line + "\n";
            out << palindromeNo;
        }
        if(in.eof()){
            break;
        }
    }
    in.close();
    out.close();
}

bool PalindromeDetector::isPalindrome(string s){
    unsigned i = 0;
    Stack<char> s1(1);
    ArrayQueue<char> q1(1);
    while(s[i]){
        char c = tolower(s[i]);
        if(isalnum(c)){
            try{
                s1.push(c);
                q1.append(c);
            } catch(StackException& se) {
                unsigned capS = s1.getCapacity();
                unsigned capQ = q1.getCapacity();
                s1.setCapacity(2*capS);
                q1.setCapacity(2*capQ);
                s1.push(c);
                q1.append(c);
            }
        }
        i++;
    }
    while(s1.getSize() != 0){
        char ch1 = s1.pop();
        char ch2 = q1.remove();
        if(ch1 != ch2){
            return false;
        }
    }
    return true;
}

PalindromeDetectorTest.h

#ifndef PALINDROMEDETECTORTEST_H_
#define PALINDROMEDETECTORTEST_H_

#include "PalindromeDetector.h"

class PalindromeDetectorTest {
public:
    void runTests();
    void detectPalindromesTest();
    void isPalindromeTest();
};

#endif /* PALINDROMEDETECTORTEST_H_ */

PalindromeDetectorTest.cpp

#include "PalindromeDetectorTest.h"
#include <cassert>
#include <iostream>
#include <fstream>
#include <cctype>
#include <string>

using namespace std;

void PalindromeDetectorTest::runTests(){
    cout << "Testing palindrome methods... " << endl;
    detectPalindromesTest();
    isPalindromeTest();
    cout << "All tests passed!\n" << endl;
}

void PalindromeDetectorTest::detectPalindromesTest(){
    cout << "- testing detectPalindromes()... " << flush;
    fstream in;
    string fileName = "testFile.txt";
    in.open(fileName.c_str());
    assert(in.is_open());
    cout << " 1 " << flush;
    ofstream out;
    string fileOutName = "testFileOut.txt";
    out.open(fileOutName.c_str());
    assert(out.is_open());
    cout << " 2 " << flush;


    cout << " Passed!" << endl;
}

void PalindromeDetectorTest::isPalindromeTest(){
    cout << "- testing isPalindrome()... " << flush;
    // test with one word palindrome
    string s1 = "racecar";
    assert(isPalindrome(s1) == true);      // these are not recognized within the scope
    cout << " 1 " << flush;
    // test with one word non-palindrome
    string s2 = "hello";
    assert(isPalindrome(s2) == false);    // these are not recognized within the scope
    cout << " 2 " << flush;
    // test with sentence palindrome
    string s3 = "O gnats, tango!";
    assert(isPalindrome(s3) == true);    // these are not recognized within the scope
    cout << " 3 " << flush;
    // test with sentence non-palindrome
    string s4 = "This is not a palindrome.";
    assert(isPalindrome(s4) == false);    // these are not recognized within the scope
    cout << " 4 " << flush;

    cout << " Passed!" << endl;
}