OpenCV VideoCapture works inconsistently - c++

Question

I have been trying to get my application using OpenCV to work for a while now and a longstanding error which I cannot seem to fix is this one:
PROBLEM:
OpenCV's VideoCapture is inconsistent in splitting video into frames for different file formats and codecs.
Specifics:
My errors are in one part of the application: splitting videos into frames. There are three current scenarios when I choose a valid directory for VideoCapture and open it. It either:
Opens and works perfectly.
Is able to open the VideoCapture (cap.isOpened() = true) but receives an empty frame or two during the frame split which causes app to throw error.
Does not open at all.
A given video will only do one of those three things. AVI File formats seem to have most of the errors, though.
Fixes tried:
Installing K-Lite Codec Pack Full
Putting OpenCV_FFMPEG.dll and OpenCV_FFMPEG_64.dll in project directory and in PATH
The only explanation I can think of is this is an error with the way I installed OpenCV and FFmpeg.
Any help would be greatly appreciated! Thanks.
By the way, I am running Qt 5.8.0 and OpenCV 3.2.0 and this is my FrameSplit code.
vector<int> mainFrame::frameSplit(string filename, string location)
{
//Extract + Save Frames
string vidDir = filename;
string locationDir = location + "/frames/";
VideoCapture cap(vidDir);
if (!cap.isOpened())
{
qDebug() << "OO";
return {};
}
else
{
Mat firstFrame;
Mat nextFrame;
Mat frame;
int count = 1;
QProgressDialog progress("Extracting files...", "Abort", 0, cap.get(CV_CAP_PROP_FRAME_COUNT));
progress.setWindowModality(Qt::WindowModal);
progress.setAttribute(Qt::WA_DeleteOnClose, true);
progress.setFixedSize(500, 100);
progress.show();
vector<int> compression_params;
compression_params.push_back(CV_IMWRITE_JPEG_QUALITY);
compression_params.push_back(100);
qDebug() << cap.get(CV_CAP_PROP_FRAME_COUNT);
bool failure = false;
for (count = 1; count < cap.get(CV_CAP_PROP_FRAME_COUNT); count++)
{
cap >> frame;
if(frame.empty()) {
QMessageBox messageBox;
messageBox.critical(this, "Error", "There was an error in analyzing the video. Please try and install the appropriate codecs and try again.");
messageBox.setFixedSize(600,400);
vector<int> empty;
return empty;
}
string filePath = locationDir + to_string(count) + ".jpg";
imwrite(filePath,frame,compression_params);
progress.setValue(count);
}
progress.setValue(count);
progress.close();
vector<int> props;
props.push_back(cap.get(CV_CAP_PROP_FRAME_COUNT));
props.push_back(round(cap.get(CV_CAP_PROP_FPS)));
props.push_back(cap.get(CV_CAP_PROP_FRAME_WIDTH));
props.push_back(cap.get(CV_CAP_PROP_FRAME_HEIGHT));
qDebug() << props[0];
return props;
}
}