I am guessing that the traffic targets are taking all the available serial port time and not letting the FIS-B through. The weather should normally show up within about 10 minutes of reception.

If you want something to try; remove your 1090 receiver and see if the weather shows up more consistently. This removed the Air to Air reception that may be clogging the serial port. To get this fixed its probably a request to the Stratux group to figure out how to carefully prioritize the serial packets so that weather is above over certain non-critical traffic packets.