Usually externals wont load for a couple of reasons
Silent running ( we have all left it in silent running before
)
Crew fatigue
Not surfaced ( torp pic should be coloured not greyed out )
Also make sure they are going into the spare slots not the actual torp tubes
If you hold your mouse over the torp you have dragged in it should give you a message
ie Not enough crew in Bow\Stern torpedo room
or Time till load xxxx etc