Most 3D printer extruder problems are caused by incorrect extruder idler tension.

Dial in the extruder tension while the nozzle is off the bed and in the air. This allows you to adjust and fine tune the extruder idler tension while it's extruding. In Pronterface, in the lower left hand corner of the window set the Extrude length to 40-100mm and the speed (right below it) to 150. Push the Extrude button and adjust the tension on the two screws (tighter/lighter) until you get consistent, repeatable extrusion.

Verify that the X axis is level to the aluminium bed platform. With a digital caliper, measure the distance from the bottom of the smooth rod to the top of the aluminium bed platform. Make sure that the measurement is the same on the left and right hand side of the bed. If it needs to be adjusted, power off the printer and rotate one of the Z axis stepper motors by hand to bring the X axis into alignment with the Y axis. Once level move onto leveling the glass bed in relation to the nozzle.

Slide a thin piece of paper underneath the nozzle and adjust the z axis end stop until the tip of the nozzle applies a firm pressure on the paper. It should not tear, but some resistance should be felt. You would then adjust the other 3 corners of the bed to match. Once dialed in you can confirm your bed leveling with this print: Measure the extrusion width, ideally the width would be the same in all areas of the bed. You would raise/lower the corner to minimize/increase the extrusion width to match the others. Once they are all consistent, the bed is level.

More 3D printing troubleshooting can be found in the RepRap Wiki