Gears tight to engage.

[Bigjcc55]

Driving the van yesterday I started having trouble engaging fourth, when you can get it to engage it stays there and changes down to third fairly easy, the box is making no noises.
Today I've checked the master and slave cylinders for leaks and all ok and removed the linkage rod and I'm having the same trouble engaging by hand.
Just drained the oil which has only benn in the box for a year and it looks like it's still fresh
Any ideas what to try before I remove the box?

Thanks

Jc

[Bigjcc55]

It was the camper not the van so it's a petrol 4 speed.