This is. a common issue, if the car is a hard top; typically the antennae base, has a circuit board in it and it is amplified. Over the years, the plastic cracks, the rubber seal deteriorates and this allow water ingress, which corrodes, damages this circuit board and kills the amplification of the FM signals but AM tends to continue to work fine. I replaced mine with a correct genuine vw replacement and my FM stations reception, was restored.
You might try some of the built in "FERN" testing procedures in the stock vw head unit and see if the system, shows the antennae system is connected. Mine passed the test but ended up needing a new base and i put a new top "flag" as well, i was back in business. I tried some of the less expensive aftermarket bases but they ended up not having the amplifier installed in them and they didn't work well, FM reception issues remained. So, keep that in mind and get a quality genuine VW or oem replacement part , for things to work correctly and restore your full functionality of your Monsoon radio , FM/AM reception. There were a number of different models of amplified nases; so it is probably best to check with your Volkswagen dealer by Vin number and that way you get the right one for particular vehicle, Monsoon head unit.