First of all you really shouldnt use old pins or barrels by that matter.. your asking for an infection.

However i could relate to this, when i first started. i was using very small needles to inject in my delts and my friend was putting pure pressure to get the juice in an the barrel popped leaving the needle in my arm and oil was gushing out.

It could be that the pin you are using maybe blocked.

still i would be carful and use a new needle and barrel every time you inject.

WBM!