Hm, it's strange. They are unlocked by completing girl's storyline. Unless you didn't finish their routes, I'm not exactly sure what might have caused this issue. You can also double-check your gallery, since they might have appeared there instead.
If possible, could you submit your save file, so we can properly investigate the problem?
Apologies for late reply
https://drive.google.com/drive/folders/1mbB2QBlSrzGaZ2Q3TC4owV6987iqkg3a?usp=sha...
Hope you can figure out the problem.