My first thought is an uneven bed that is too close to the extruder. But I am pretty sure the bed is not flat. There is a low spot right in the middle. If I raise up the extruder any more the plastic will not adhere in the middle. With medium to large prints this happens on both sides of the bed. The extruder gets too close to the bed and prevents plastic from coming out. The stepper makes a ticking sound when this happens.
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