If your AIS receiver seems to be working but you are consistently missing vessels that other stations are seeing, your antenna is probably not the problem. Neither is your SDR...
There is a persistent myth in GPS and GNSS system design: more gain ahead of the receiver is always better. Stack enough amplification in front and the problem goes...
Your GPS receiver is not broken. The satellite signal is fine. The problem is sitting between your antenna and your chip, and it is almost certainly caused by a signal...