This guide shows you where all the mods show up in the game and from which store you can buy them. If you're doing a newgame+ remember, that you cannot level them beyond V. Other than that - if they don't show up at all it looks like your gamefiles have become corrupt, so you either try a repair install or re-install.
What platform are you on? I've heard of these issues but not for PC.
I'm on xbox,
the thing is i can see some if them, i can click on them to make them disapear, but they just never show up in inventory or in shops, so for 2/3 of my games i have no sniper extended barrel of any kind
As far as I know this is a known bug, but I don't think it's been fixed. Having said that I'm a PC gamer so don't follow xbox issues all that much so could be wrong.
I can confirm this issue on PC as well. I've basically not been able to take/keep all weapon-mods I found.
Started right on priority:Mars. First I was able to take the SMG-mods in the beginning; then I died and used the restart-mission-save. From that point on, only the animation played while pressing space on the mods. The sniper-rifle-mods worked, though. Same problem on the Grissom-Academy; When I first played it I was able to take the mods lying around there, but when I had to reload it was animation-only again, with no window popping up. Meaning those mods aren't available on weapon-bench or shops, which made me stop playing 'till it's fixed.
I've tried scanning for corrupt files, but Origin says all my game-files are fine and up-to-date. Same for all DLC-contents. Also tested on multiple careers with normal and extreme difficulty (both with imported character from ME2 which also was carried over from ME1), same issue.
