One of three things.
Float level too high
Something stuck in the needle
Too much fuel pressure to carb
Has the car been sitting for any length of time? If so, it may have crud in the fuel lines and after rebuilding it the carb may have gotten more crud on start up and plugged the needle again. Had this happen on one that I rebuilt on a car that sat for 10 yrs.
Pull it apart and recheck everything, then make sure you've got a good filter ahead of the carb.