Toilet won't completely flush

Check the jet to see if they are clogged with calcium. You can clean them with a metal clothes hanger but be careful. https://youtu.be/3ohFlKyOLPA

My main toilet is a water-saver model but over 25 years old. We hold the handle down for a full three seconds to flush. Check inside the tank to make sure the water tube is connected to the ceramic pipe inside. Check any chains, flaps, floating balls, metal chains to hold the chains, etc. to see if anything is rusty, gummy, moldy, or I'll-fitting. Also see if a brick or other object has been put into an older bigger toilet to displace water--that's what we used to do as a water-saver. By the way, unless I am cleaning the tank of mold or minerals, I do all this checking with water in the tank. If it grosses you out, turn off the water to the tank, flush once or twice, and the insides of the tank are accessible without water in your way. Then refill the tank by turning the water valve on, and flushing and waiting for the tank to refill completely. Make any needed repairs, but but be aware: the newer the toilet, the more likely you have a water saving model that limits the amount of water that flows into the toilet bowl. Best wishes 😇

Sometimes the flapper closes too soon. You can adjust it to close later.

two possibilities are the flapper closes too soon, adjustable by the chain that connects it to the handle, or there isn't enough water in the tank, adjustable by the float in the tank, or the toilet is a water saver as described before

You need a new rubber flap, or the hose in the tank is not filling up completely. When you flush the tank have the top off to see what's going on. You can get a replacement kit at the hardware store. Let us know if any of the suggestions work.