Extruder servo only performs 3 movements

I have a RipRapPro tricolour machine and just commissioning it with the kits own controller. (So learning about it!)
All servos perform as expected using the Pronterface Programme save for the extruder servo (which upon some investigation) will only work for 3 operations, either extrude or reverse before stopping and locking on. This servo will only work again if the controller is switched off and then switched on again. I have tried it several times and it is 3 operations then locking. I have tried disconnecting and reconnecting the computer, but that has no effect. The board has to be powered down. The other servos work fine
Has anybody got any ideas if it is the board or software,or anything else???
Regards

I forgot to add that I have tried another servo, and this performs exactly the same.

i have put one of the output legs on an osciliscope and there is no steps being transmitted after the 3 operations of either exrude or reverse.

Being new to this game. and having expermiented and taken the extruder to pieces, replaced the servo, re-downloaded pronterface, used another computer, plus another copy of ponterface, I think that what I was trying to do was to extrude more than 3 times in a cold condition , which I would suspect, the software does not allow.
I reassembled all the bits together again and had a good print, so if anybody else does their testing and thinks that there is something wrong, (unless otherwise corrected) I think you need to press on as the rest seems to work OK.

Was the filament loaded when you ran the extruder with the hot end cold? If so that's a no no. : )

We all start out learning stuff the hard way like this. If you haven't done so already I encourage you to read the reprap page for your printer model and each if the pages for the specific parts if it. Top to bottom. I also reccomend you come here to read this forum on a regular basis. I think a lot of people only come here to post when they have an issue.

My piecs of immediately useful advice are:

• Don't skip over calibration.
• Learn about v-ref for your stepper drivers.
• Make damn sure your power supply is up to snuff. I only have a single head machine and I suspect that the accepted standard if 16 amps doesn't hold true. My printer didn't run right untill i added a second supply.

Appdev007- Thanks for that.
For others information, I did not have the hot end connected as I wanted to extrude the filament into open air to see what distance was being sent and compare, the designated computer control distance with actual. When after 3 extrudes (or 3 reverses or any combination) does not do anything more, then the components/ingredients get questioned.
Experience!