Two points:
- "Rubber" hoses seem to "weld" themselves to the push on spigot/barb. The older the connection & hose, the harder it is to separate them. The advise about replacing the hose is good. Go for high quality hose, as the hardening over time will be slower ie removing your filter will likly be easier. Note: Even unused hose, if kept for long enough, will show signs of hardening.
- ILENGINE's prybar advice is spot on, if a tad expensive for something you can easily make yourself, out of an old or cheap wide blade screwdriver.
I find using a smear high quality silicon grease (no hydrocarbon) on the spigot/barb, before pushing on the hose, will make for an easy push on and easier removal, at the next service.
If the hose is really stiff, it will be easier to remove, if you slit it over the spigot/barb fitting.