This could be happening because Google might be trying to curate your photos and just display what it thinks are the best.
The way I avoid this is by creating a Google photos folder especially for display and marking it as a favorite or turning off 'Personal photo curation'.
More information about this can be found from the link below.
Let us know if you have any further questions.
Thanks for visiting the Community.
@Cathal_S, thanks for helping out answer this thread.
@ Larryfromupstat, hope you got the answers that you're looking for with the info provided on the previous post. Feel free to respond if you have additional questions and the Community would gladly help.