Logic Pro 9-sometimes audio files are not playing

I am working on an old project in Logic Pro 9 on a 2011 macbook pro X 10.6.8. I cut up and pasted samples I made of a drummer on various files on various tracks. They are occasionally playing and then occasionally not playing. This only seems to happen during consistent sections of the song but not to consistent files. Sometimes when I begin playing from where these inconsistently playing files are located, a few files play and then the rest do not. The same files are cut and pasted on other tracks and work so I don't think they are corrupted. I adjusted the latency, changed the buffer size to 1024, etc, then the plug-in latency and switched on low latency mode, etc. Changed nothing. Universal track mode is on. None of the files are muted. The only plug-ins are compressor and linear phase eq and pitch shifter on each track and a compressor and match eq on the bus. The files play fine in the hyper editor. Panning is adjusted but that doesn't seem to affect it. All the tracks are mono. The bus is mono. The song is 20 minutes and very complicated. Perhaps this has something to do with it. Maybe it's the old computer. Thanks for any advice you might have.