At 1:18:35 you pick up a Shotgun Shredder mod, but you didn't get a pop-up in the lower right, meaning the mod will not appear in your inventory. This is a well known bug that was never fixed. However, I think that you can buy all the weapon mods you missed as soon as that mission is over from various shops (unlike ME2, where if you miss an upgrade, it's gone forever). I wasn't able to confirm this as some people online said they appear in shops and some people said they didn't.
Personally, I just ended up using a mission guide so I knew which mods were available in my current story mission and then added them to my inventory with a save editor as soon as the mission was over.
Weapon modding and trying out different guns/mod combinations is a lot of fun in ME3. You'd be missing out if you didn't have access to all the weapon mods that are supposed to be in your inventory. Might be worth looking into a solution....
DukeEdwardI's comments