cocking problem

[contact148]

IM in the process of acquiring my 1st luger don't own one yet can get this one for a few pretty cheap but I want it to work!

Guy at my club has a ww1 1917? IIRC all matching except side plate and magazine is missing (story was it got hidden then lost/forgotten because of kids) he got a side plate (sarco) we installed it and the gun is not cocking every single time

with snap caps, action open drop a round in chamber pull toggle release and NOT cocked 95% of the time. BUT load from magazine it cocks 99.9% of the time

Gun is clean and oiled trigger bar plunger is free and works IE trigger pulled work toggle release trigger gun "fires"

I'm pretty sure its side plate related but cant be sure how do you make sure side plate fits correctly? ive seen that they need to be "fitted" plate is tight on gun trigger moves trigger bar when pulled. seems to me the firing pin is not being held by the trigger bar every time nothing looks worn don't think this was shot much

we have not shot it yet hopefully this weekend if he is able

thanks