I'm trying to pocket some text on a piece of formica, but as the cutter works along the piece the depth of cut is getting shallower & shallower, for example we're cutting to 0.6mm depth with 6 passes, the first few letters start cutting on the first pass, then the next few takes 2 passes before it starts cutting & so on until nearer the end it's taking 5 or 6 passes to even start cutting & sometimes not even touching the workpiece towards the end.  The work piece is about 6" in length.
I've gone over the work surface with a bottoming bit so it's not an uneven work surface,
Any ideas what the problem might be?
Sorry if this is a bit vague I'm pretty new to CNC

After some more research on the internet we've decreased the velocity and increased the acceleration on the z axis so
original settings were: steps 400, velocity 1500, acceleration 400 (as directed by manufacturer)
Settings changed to:   steps 400, velocity 1000, acceleration 600 (plucked out of thin air)
We've just plucked these figures out of the air but they seem to have sorted the problem out. Can anyone advice as to what the velocity & acceleration Should be & should we change the  x and y axis settings to this as well? All where set at the same 1500 velocity & 400 acceleration.

Going by the spec on the Ebay advert the stepper motors are: 57 two-phase 3A 150N.cm (doesn't mean anything to me though)

Can anyone advise to what what velocity & acceleration figures I should be using? these ones I've plucked your of thin air seem to be working fine, but I don't want to be doing any harm to anything.

You won't be doing any harm to anything - if those settings work reliably than stick with them.

Tweakie.

 THANKS for the reassurance Tweakie, it's much appreciated.

We still seem to be having problems with missing steps on the Z Axis!
Does anyone have any solutions?

There is no way for us to tell you what is your problem.  Each machine, even of the same model, may have different characteristics due to shipping, assembly or usage. 

Most likely reasons are binding on axis, tool not being held tightly (debris in collet prevents full closure), motors too small for load, loose coupling on screw.  Then you get to too much acceleration or too fast for the stepper to handle it.  No magic formulas here, sorry.

Search for motor tuning acceleration or missing steps and you will get a plethora of possibilities.

HTH

RT