Probably because a lot of the spares are badly made. I had a sear for a Premier (thread on here) that needed an ANGLE GRINDER to take enough off the bottom to enable it to cock.
You should notify Nibbs and send it back an hopefully the replacement should fit. If people don't notify them of problems, they will carry on getting and selling the same poorly made parts. It does not say that it requires some fitting so it should be fit for purpose
http://www.airgunspares.com/store/pr...y-Senior-Sear/.
If you want to have a go at it, then remove the mainspring and push the piston back to see if it engages. (It probably wont, but if it does, the chances are the holding face of the sear needs a little taking off it as it may be too thick to enable the piston to engage OR the edges of the sear are too wide---does it slide in the trigger housing ok?).
If it does not cock, then take out the trigger and try it again. If it does not cock then, then it is probably the sear face.
If it does now cock, you have to look at whether the sear will jam on the trigger or the trigger is jamming on the trigger spring or the housing.
All this is easier to do with the mainspring removed.