Toilet weak flush. The water in the bowl swirls, but it didn't flush stuff into the pipe
There are 2 things that could be causing your problem. A partially plugged drain slowing the water. Or a plugged vent that is not allowing the water because a vacuum is being created.

For me, when I have a slow drain like this I will use a toilet auger to make sure the toilet it self is clear (had a kid flush a dead light bulb and did not find it until using the auger, which broke the bulb).

If the auger makes no difference, I pull the toilet. It's not that hard, just a bit messy. Turn the water off, disconnect &amp; flush the toilet. A shop vac is handy here to remove reaming water. Remove 2 bolts anchoring the toilet to the floor. Lift the toilet, and now you can see the drain and possibly the wax seal on the flange (or on the toilet).  I use rubber gloves and a small putty knife to scrape off the old seal. Was it centered?  It could be the problem if not centered.

At this point I run a snake down the line, and I also examine what I can see. Has somebody poured grease down the toilet? Found that quite a few times, but usually the snake fixes the problem. In rare cases, I have found a dead squirrel in the vent line and a bird's nest but a plugged vent can cause problems also, but usually other drains in the bath also have problems if this is the case.

Once the line is snaked clean, I press a new seal on the toilet (I like the ones with the urethane core but a solid wax ring is cheaper, just a few bucks). Place the toilet back on the flange and put the 2 anchor bolts in. Connect the water and give it a try.

Chemical drain cleaners usually don't work well on toilets because of the large volume of water and if the water is slowly draining it won't be in contact long enough to do any good. After my troubles with a flushed light bulb I now auger the toilet because that one cost me hours to figure out. The bulb was stuck in the toilet trap. And water would run slowly but not flush.