Is it a solid pin, or a rolled-steel (roll) pin?
In general, a solid pin is usually held in place by a tight friction-fit with the hole, or the ends of the hole are "staked" (partially collapsed; something like a center-punch dent near the edge of the hole to make the hole smaller/tighter and not round) so the pin can't start sliding out. Sometimes a solid pin can be held in place by another part (or parts), or the sides of a slotted part or frame. In the case of a pin passing through the trigger from side-to-side, if the axle pin is installed in the trigger first, then the trigger is installed in the frame slot, and the ends of the pin never drop below the slot or rise above the slot as the trigger moves through it's arc, then the slot will hold the pin in place (although it may drag, adding friction and increased pull-weight to the action).
A roll pin is held in place by the fact that it is compressed during installation, so it fits tightly in the hole. For this reason, roll pins are usually replaced when they have to be removed, vs re-using them (once they have been driven in and out a few times, they no longer fit tightly enough to stay in place).
Not being familiar with this exact pistol, I can't say for sure why the pin isn't staying in place. If you were to take it to a gunsmith and explain the problem, they might be able to fix it quickly and easily (but they still might charge you $50-$100 or more for the repair).
In searching for this pistol model and the words "problem trigger axle" I found at least one other person who was experiencing the same problem. He expoxied (glued) his axle in place. I would NOT recommend that as a reasonable action. There are reasons manufacturers don't often use glue/epoxy on gun parts.
If you don't have a local gunsmith available to work on it, then I'd recommend sending it back to the manufacturer for a warranty repair.