Advice sought on ways to stop a chisel jamming in wood when using a
machine morticer

I am morticing a fair few holes in treated pine (to make farm gates)
The wood is probably still a bit damp from the treatment.

However whilst doing the 1st cut (into wood that is all around the
chisel), the chisel always jams solid in the wood. It does not matter
how deep I cut and withdraw. It still jams, and I have to knock the
wood off the chisel.

I cannot use the machines clamp, as the wood is too thick.

It mortices OK with the next cuts that are adjacent.

Has anyone any constructive tips that could help, or do I just have to
grin and bear it.