Check around the area and make sure that there are no hairline cracks. If there are no visible, it is likely that the filter has not closed far enough:
1. Make sure that the thin, black O-ring is present and sits flush in the channel just below the threads on the larger half of the housing
2. Try to screw the housing pieces tighter together. If you feel that you have gone as far as you can, apply a smear of Vaseline or coconut oil to the threads. This will help the threads move more easily. You could also try using petroleum jelly to help seal the housing and prevent mineral deposits from accumulating between cartridges changes.
3. If you have gone through the steps above and still have leakage, especially after a cartridge change, remove the cartridge from the housing. Make sure that there is no old gasket from the previous cartridge still in the housing. Use only the gasket that comes with your new cartridge.