Fitting mower deck wrong?

[BritHorse 0]

I'm quite a new Wheelhorse owner here in the UK. I have a 227-5 Europa tractor which I want to use mainly for grass cutting. I've used the deck which came with it a few times so far (it's a 42-inch rear discharge version, I think, and it has the long flexible cable to raise/lower the front of the deck!) It works...kind of, but I have real trouble getting it set up right, so maybe some one out there can help with advice - please!

The problem is the drive/pulley under the engine and the main pulley on top of the deck which drives the spindles/cutting blades don't line up. I can get the belt on OK, but when I lower the position of the deck for cutting, the two pulleys are at different angles. After a while it either throws the belt off, or starts to shred it! Can someone tell me what I'm doing wrong? Thanks.