P 350 problem

[DFBstella]

I've been having a problem with the fore-end not locking out all the time. Sometimes when I load it, putting two shells in the bottom first, then pumping it and putting a third shell in, it won't lock out after the third shell. If I load it by putting a shell into the ejection port first, then two in the bottom its fine until I shoot one. Then I load a shell into the bottom again and it doesn't always lock out. Could this be a mechanical problem with the gun?