Hello Roofiern and Welcome to the Forum:
Yes, the water level in the tank could be leaving some residue. Check the level and set it to 1" below the overflow. There is also the possibility of an obstruction in your house drain or sewer.
If changing the water level does not cure the problem right away you should use a "closet auger" to be sure the commode and closet bend are clear. Then, if that doesn't get it, rent a manual snake to locate the obstruction and it might possibly knock it loose. If it doesn't knock out the obstruction you can rent an electric snake and reem it out. Caution: electric drain clearing equipment is dangerous, handle with care.
If it is a blockage down stream somewhere and you are able to clear it, you will probably find roots collecting wet wipes or feminine napkins. To rid your systme of roots get a bag of salt pellets for a water sofetner and flush a cup of pellets once a week or so. The roots will trap the pellets and they will lay there and make a brine that will kill the roots out, this may be an ongoing procedure. Let us know how it turns out.