gr-ais | Automatic Information System decoder for shipborne position | SDK library

I'm having trouble getting this to work: https://github.com/trendmicro/ais

The software contains a GNURadio Companion flow graph that sends AIS Messages to a USRP. I've installed the included gr-aistx library, and everything seems to run smoothly. However, whatever messages I may be sending don't seem to be getting picked up by an AIS Receiver.

I've tried the following. As a note, I am not transmitting/receiving over the air. Every test connection is hard-wired from the transmitter to the receiver:

• Transmitting AIS using the included GRC flow graph. This flow graph seems to repeatedly send out messages on alternating channels. The flow graph looks something like this:

  AIS Frame Builder (custom GRC block) -> GMSK Mod -> Multiply Const (0.9) -> UHD: USRP Sink

• Transmitting using the phython scripts included in the repository(AIVDM_Encoder and AiS_TX).

• Transmitting using both a USRP (Ettus X300) as well as a HackRF. To get the HackRF to transmit using the GRC or scripts, I had to swap out the UHD: USRP Sink for an osmocom sink after installing gr-osmosdr.
• I've got access to an AIS Receiver which is hard-wired to the transmitting radio. No messages appear to be picked up by the receiver. I've verified that the AIS Receiver is working correctly by pumping it messages from a Shine Micro AIS Test Set. https://www.shinemicro.com/product/st162-t1/
• I've also tried receiving using gr-ais's ais_rx script. (https://github.com/bistromath/gr-ais). I installed the software and receive AIS messages through the HackRF. When testing against the trendmicro software no AIS messages are picked up. I have also tested gr-ais using the Shine Micro Test Set to make sure its working.

I'm not entirely sure what the issue might be. The flow graph seems pretty straightforward but I am in no way an expert at digital signal processing.