Is the spring that controls the pawl properly installed and in the right position? I know there's a slot that the pawl has to ride in inside the frame; and, the transfer bar has to be properly aligned, too, before the pawl will slide into its recess.
Here's what my expanded Ruger Manual says:
"Hook the front of the trigger guard into the frame recess. Align the pawl in its recess inside the frame, and be sure transfer bar is in the correct position inside the frame. Then allow the latch to re-engage. If solid resistance is felt when swinging trigger guard into frame, or if trigger guard will not lock into position, then recheck the position between the pawl and transfer bar."
I think this is where you're having a problem. One last thing: Try lightly pushing on the side of the pawl. Sometimes it isn't properly seated on its pivot pin. Sometimes the spring is off, too. Check everything carefully, and make sure the alignment between the pawl and transfer bar is correct.
If the parts are properly assembled then the pawl should go into its frame recess without a problem. If, however, you're missing a spring or plunger underneath the pawl, then, it's not going to function correctly.
Ruger SP-101 Schematic
Double click on this schematic and it will open up to reveal the correct relationship between the parts.
PS: I've had this problem, too. I solved it by carefully reassembling the pawl components and making sure that everything lined up before I reinserted the trigger assembly back into the frame. It can be tricky.
You haven't lost anything like parts #49, or #50 - Have you?