"why is it that in the months leading up to the election the JLP seemd so far ahead in the polls yet at election time they are not able to crystalize that support?"

Leadership and organizational problems.

"if this is the case how was the JLP able to show a 16 seat gain in the last election dispite the leader"

Because PNP voters stayed home, not because there was any "swing" in favor of the JLP. Mi naah go into the maths again, I already did that with YG a couple pages back.