Majority of pipes that I use are with 9mm filters. I identified three areas that can create a problem in these pipes. They are construction, filters, and maintenance. The construction issues are most important in my view but it is wast area and I might discuss it some other time. Filters and maintenance are easier to address.
Filters - the issue here is that only branded filters are designed properly and the appropriate material is used. I prefer filters made by Vauen, the designer of this system. The worst filters are made by China and they are sold on eBay.They are all branded as Vauen but they are not made by this company. They are fake. The original Vauen filters are made with a front cap cut in ceramic. This is very important! Ceramic with withstand the temperature in the shank and function properly. The fake filters made in Chine have a cap made of plastic and it will melt during pipe smoking. Use only Vauen filters.
Maintenance of the pipe - I found that daily cleaning of the pipe that uses 9mm filters is crucial. The filter stops travel of tar into the stem. Hence, tar accumulates in the first section of the mortise. If that accumulation is not cleaned often, tar will dry and clog the mortise and the filter. With time filter will be stuck in the mortise and this is what probably happened to you.
Here is the image of pipe parts for easy identification.