If it works sometimes, and not others, and it doesn't work for different reasons, it seems to me that there is something going wrong in the execution of the code.
For test purposes, create a little script which is called when the trigger fires. Get the instructions into a good order, so that only one thing is happening at a time. I know it shouldn't make any difference, but as you know OFP is a fickly mistress. LEave the waypoint on defaults of normal speed and aware, till its working reliably, then mess with stuff like that if you really need to.
;script sketch
"_x moveInCargo URALS" forEach units REgrp
~3
"_x assignAsCargo URALS" forEach units REgrp
(can help avoid problems when getting them out again)
~3
URALS setPos GetMarkerPos "UrSM"
~1
URALS setFuel 1
~1
papabear sideChat "INTEL REPORTS Blah, blah,blah."
Obviously test it with yourself as ural driver and leader/member of the group.
I know all that doesn't really amount to an answer, but it might help. The only other thing I can think of is that the spot it is setpossed to is difficult for the truck for some reason: try moving it a few yards.
And welcome back!