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I've got a Rostock Max V2 here and I'm having terrible problems printing anything! It worked just fine for one or the other right at the beginning but now I have nothing but trouble printing. More specifically, my extruder can't create enough force to get the ABS thru my hot end which I set to the max temperature of 240°C. While printing I hear a constant clicking coming from the extruder - The filament is popping back up and or eating holes into the filament!
A friend suggested it could be the bowden tube which is causing a lot of resistance, but when I unscrew the tube fitting from the hot end there is no force needed to operate the extruder. I did notice however, that the ABS filament is getting stuck in the PTFE tube inside the hot end. The diameter of the filament which is getting stuck inside the tubing is 2.15mm in thickness (My filament is pretty much 1.75 unmelted) and the tube itself has an inside diameter of ~2.0mm. I do understand that when heating the hot end, the tube inside will expand.
I'm pretty sure that the hot end is up to temperature, as if I hold some plastic against my 0.75mm nozzle, it will melt as you would expect it to without any force applied!

I'm not sure if the problem lies with my hot end or if I'm looking in the wrong place.

What hotend are you using?
Are you actually using a .75mm nozzle?
Have you tried to increase the current to the extruder motor in the firmware?
Have you calibrated your E steps for the extruder?
If you remove the Bowden tube from the extruder and feed the filament into the tube by hand does it move through the hotend fairly easily or is a lot of force required?

I'm using the stock hotend which came with my rostock printer. I used a 0.35mm nozzle in the picture above, but switched over to the 0.75mm later but haven't been able to print anything at all with it. I think the extruder already has enough current as you can see in the picture below. It's actually "eating" holes into my filament. I had another look at my E-Steps setting and it seems to be slightly off by 0.1. I have corrected it to the 92.65 as noted in the manual.

If I take the bowden tubing off and tell Matter Control to extrude 100mm I also get 100mm out. I don't think the problem is with my extruder. While printing I do hear a clicking noise every now and then - it's the filament which is popping back up thru the extruder as it can't hold the fore back and thus eating holes into the filament. It has actually cut the ABS in half before! There is basically no force required to push the filament thru the bowden tube by hand.

This problem requires more brain power and I ask other users to think about Birdmonster's problem and jump into the discussion.

If you can't push the filament by hand then your hot end is cold or clogged.
If you have a multimeter measure the temperature.

I currently don't have a multimeter with a temperature sensor on it. I'll try to get one this weekend, however if I hold some of my filament against the outside of the nozzle it will melt instantly. I could imagine that it's clogged, but i have cleaned it out a few times already! I'll give them another clean tomorrow and try it again.