Hi: Try these things. I had this problem starting around 2005 with boots that were several years old.
1. Buckle boots up as tight as they go and let them sit for at least one day. Lot of folks leave the buckles open for storage causing the shell to lose its shape.
2. At home put your normal ski socks on inside out and then try your put your foot in the boot. Then wear boots around the house for a bit The liners should pack out to last years comfort.. I found out about this sock trick by wearing socks inside out by accident. Wearing the sock inside out slides the foot in the liner nicely. Do not attempt to ski with your socks inside out because your foot will slide around in the boot, I ended one ski day early because of feet sliding made me clamp buckles too tight causing numb cold feet.
3. Make sure boots are warm before you put them on, I used to go skiing right after work and snuck the offending boot in to my workplace and hid it in a nice warm closet This worked great for one season until found out part of the problem was my foot which leads to suggestion #4.
4.See a foot doctor and/or assess your foots range of motion, Perhaps your range of motion of some part of your foot or ankle is getting slowly limited making your foot stiff. The lst sign of arthritis can be a gradual stiffing of a joint. This stifting came to me 1st got worse for several years and then got painful. I ignored this for several years and now my big toe joints are trashed. Arthiritis and old injuries are very sneeky and can cause problems in the foot and ankle.