Help. Xbox, haven't worked out how to equip the secondary shell in the MBTs

I just don't get it, I definitely have the secondary shell selected in loadouts, with the green tick, but when I play I only have the primary shell available.

I tried AI but it didn't help at all. It stated it was the left trigger button but that doesn't do anything and I also read your 20 shells are split between both but I just have the usual starting amount of 20.

Any help appreciated!