New printer motors only squeal wont move

just completed my Prusa i2 and powered it up for first time and it wont move the heat bed and hot end work and when i go to move the motors it just squeals i tried using pronter and repetier but it made no difference turning the pots only makes the squeal worse i checked the wiring but it seems right (Red,Blue,Green,Black) and all the end stops work belt tension is fine
i am using a ramps 1.4 and Ardunio mega 2560 boards
the motors are nema 17 bipolor

Help please!

Are the jumpers fitted under the pololu drivers?

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Waitaki 3D Printer

yes the jumpers are on under the pololu drivers

I had this problem on my Sanguinololu and it turned out to be poor connection between the stepper motor cable and the board (they were using these 4 way connectors). After wiggling the connectors about the motor would suddenly spring into life. So I ended up putting some solder on the pins to make them thicker to ensure a good mate.

i will try this tomorrow thanks