AVMutableComposition Black Screen on Replay After 3rd clip - objective-c

Question

In my app I am attempting to merge video clips dynamically. I have two properties which hold the previous recording and then the next recording to be appended to the end. This is basically acting as a pause function, and then the user can play it back on the fly.
I make the app write the first clip to "video.mp4" in the docs directory. This is set as the previous recording. Then I write the next clip to "video2.mp4", set it to the next recording, and merge them together using AVMutableComposition:
AVMutableComposition *mashVideosTogether = [AVMutableComposition composition];
NSError *error;
if([mashVideosTogether insertTimeRange:
CMTimeRangeMake(kCMTimeZero, [self.previousRecording duration])
ofAsset:self.previousRecording
atTime:kCMTimeZero
error:&error]) NSLog(#"Successfully added one");
else NSLog(#"error: %#", error.description);
if([mashVideosTogether insertTimeRange:
CMTimeRangeMake(kCMTimeZero, [self.nextRecording duration])
ofAsset:self.nextRecording
atTime:CMTimeAdd(kCMTimeZero, [self.previousRecording duration])
error:&error]) NSLog(#"Success on 2");
else NSLog(#"error: %#", error.description);
This appends the first and second videos. I then export the video to "combined.mp4", and when this is successfully finished I then delete the file at "video.mp4", and export the combined video to "video.mp4" (so at this point the combined video exists in two places). This plays fine in my player. If the user clicks record again, the newly combined video at "video.mp4" is set as the previous recording, and the newly recorded clip is set as next recording, and the whole process is repeated. They are appended and exported all over again to repeat the process.
However, once I add a third (or more) clip, the first clips from the created composition go black in the playback. Their duration is still kept, but there is no video or sound. Basically, anytime I create a new composition from an old composition, the first composition is presented blank, and then the only thing that is preserved is their duration, and the new recording. Is this data lost when the composition is then made into another composition? Do I need to manually add them as tracks? Any help is appreciated!
SOLVED
I read over Apple's AVEditDemo and it seems as if my original assumption was correct - when I was appending videos together using AVMutableComposition alone (AKA not creating individual track files and merging them), the data for these tracks was lost when they were added to another composition.
So I just created individual tracks for audio and video of every clip to merge them and now I have a working setup where I can shoot videos dynamically, stop, then begin shooting again and they will be concatenated on the fly.
if(self.previousRecording && self.nextRecording) {
NSArray *assetArray = [NSArray arrayWithObjects:
self.previousRecording, self.nextRecording, nil];
NSURL *fileURL = [self getURLWithPathComponent:#"combined.mp4"];
AVMutableComposition *mashVideosTogether = [AVMutableComposition composition];
NSError *error;
CMTime nextClipStartTime = kCMTimeZero;
AVMutableCompositionTrack *compositionVideoTrack =
[mashVideosTogether addMutableTrackWithMediaType:AVMediaTypeVideo
preferredTrackID:kCMPersistentTrackID_Invalid];
AVMutableCompositionTrack *compositionAudioTrack =
[mashVideosTogether addMutableTrackWithMediaType:AVMediaTypeAudio
preferredTrackID:kCMPersistentTrackID_Invalid];
for(int i=0; i<[assetArray count]; i++) {
AVURLAsset *asset = [assetArray objectAtIndex:i];
CMTimeRange timeRangeInAsset = CMTimeRangeMake(kCMTimeZero, [asset duration]);
AVAssetTrack *clipVideoTrack =
[[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
[compositionVideoTrack insertTimeRange:timeRangeInAsset
ofTrack:clipVideoTrack atTime:nextClipStartTime error:nil];
AVAssetTrack *clipAudioTrack =
[[asset tracksWithMediaType:AVMediaTypeAudio]objectAtIndex:0];
[compositionAudioTrack insertTimeRange:timeRangeInAsset
ofTrack:clipAudioTrack atTime:nextClipStartTime error:nil];
nextClipStartTime = CMTimeAdd(nextClipStartTime, timeRangeInAsset.duration);
}
//do exports down here and then reset previous recording, etc.
}