I have a toilet which has a slight slow leak at the bottom. I had this issue since I bought the house. I have replaced the ring once seemed to fix but now leaking again, 5 yrs ago. When I replaced the seal I noticed the pipe into the slab was short. Could this be the issue our is it that every 5 to ten years I need to reset.

I noticed the pipe into the slab was short.
What pipe would that be? The horn from the bowl? Is the flange below the floor line? Please explain. Tom

Flange below the floor, I have also noticed the floor may not be level and had a recomd' to shim the toilet after getting a double wax ring set and shim and the caulk. What do you think

They also make riser rings for the flange. Looks just like the top of the flange without the hub.

Install spacer / extension rings (Home Depot) to bring the flange up with the finished floor. Use fat wax ring with flange. You can also use another, skinny, wax ring on top of the fat one. When setting toilet, you should feel how it hits the top of the wax rings and how it compresses it.

flange below the floor, As the other experts suggested purchase a set of flange extenders, (See image) and a wax ring with a funnel, (see image). Good luck, Tom