Router issues

I'm having difficulty with a Trend T9 1/2 router fitted with a 50mm straight cutter. This is meant to have a 75mm plunge but I can't get deep enough to cut through 40mm oak worktop once the router is sitting on the the 18mm jig.

Am I missing something? I'm wary about pulling the cutter too far out of the collet in case it comes out when I'm routing.

Any comments gratefully received!