I cannot get this trimmer to trim cases to any  consistent length and the case mouth is not level. What am I doing wrong?

Just what I was going to ask. It sounds as if you don't have a pilot or the correct pilot and it is allowing the case to wobble. 

Here is what works for me:
- place the base of the case in the collet and slide the pilot into the case mouth.
- push the cutter blades up tight against the case mouth and loosen the collet to allow the case to seat fully into same and then tighten. This step insures the case is straight in the trimmer.
- pull the cutter blades back just a bit to allow you to turn it as you feed it into the case to do the trimming.
Works like a charm.
BTW - if you purchased used there is a chance that you need to have the blades sharpened (sending mine in tomorrow after 4yr of use). Think Forster charges $9, which includes return shipment.
If you are still having trouble post a pic of your set-up and we'll trouble shoot for you. 

Measure your rim diameter. A few years ago I was trimming some 40-65 brass and it was coming out different lengths. I tracked it down to the collet being different distances because of different rim diameters!! I bought an RCBS that holds the rim against the tool head. Yes, there could be a difference in rim thickness, but the back of the cartridge to the front end is always the same.

I have problems with my Forster case trimmer when doing the large rimmed cases. Thinking about buying a better trimmer. 

          Joe.

In my Forster case trimmer, the collet does not move, the mouth of the collet abuts the inside of the collet housing.  Only the collet clamp moves at the case head side.  So with my Forster trimmer, cases with slightly different diameters will not be trimmed to different lengths. 

I've been using a Forester for as long as I can remember and never had a problem with it with any caliber until I attempted to set it up as a dedicated case trimmer in a small import bench type drillpress.  To make a long story short, it turned out that the stop collar that limits downfeed on the drillpress was the culprit.  Even if you have used this setup before, I would suggest careful scrutiny of the spindle and downfeed limit device.  I replaced the flexible stop collar with a much more rigid shop made part and now all is good. 

Hayface