Spartan Router - Head drop issue

[Abstract]

I ran the air cutting test a couple of times to see what was happening. The cutter did not seem to drop depth from the collet.
Once that was done, I thought about having a look to see if the material was ,in a way, pulling the cutter from the collet. So I cut 3 lines in some 19mm foamex with the V-groove tool used on friday. I started at cutting depth; 1.5mm, 2.5mm and finally 3.5mm. The head did not drop after all grooves were done.
With a bit more confidence, I risked putting some work through. I started with the basic route lines (which is where it messed up friday) and it seemed to complete it without a problem. The only thing I noticed was that the corner nearest the mechanical origin seems to cut lower than anywhere else on the board; why could this be?
Also, another question - how often should I clean out the collet and locking nut?