Photomerge in photoshop what am I doing wrong???

[Martina]

I've been playing around with photomerge in photoshop tonight and I'm really swimming in the deep end, the example I show is number 5 of tonights attempts and it's obvious something is wrong with the seam accross the pic. other pictures I've attemted have been divided in 1/4's
I'm sure I've used photomerge in the past without this problem

This example is only 3 exposures, others I've tried have been 3-5 with similiar results.

so what is going on am I having a blond moment?