Project plays channels but there not exported

Hi,
Have you ever come across an issue where your project would be playing all the instruments as it should, the channel on the mixer is routed to stereo out but when you export the track it plays the sounds as expected but then all of a sudden they drop out? so for like the first 3 minutes of the track it play the sounds as expected but they all of a sudden drop out even though they dont drop out on my project…

My bass is basically disappearing even though i can see it in my project and hear it…

frustrating…
thanks

its like, i build the first half of my track and if i want to copy and paste it for the second half… even though everything copies on the project and i can hear everything… theres parts of the song that are missing from the exported wav … its so weird…