Toilets that do not flush properly or completely can be a troublesome problem. Unfortunately, toilet bowls that do not flush completely can be the result of a variety of problems. You will need to go through each step to properly diagnose the toilet's problem. Once diagnosed, you can then take the corrective actions necessary to rid yourself of this annoying problem. However, if you cannot correct the issue yourself, you will probably need the help of a professional plumber to correct major plumbing issues.
Pour a bucket of water into the toilet bowl. Clogged toilets will not flush with the added pressure of the water. Plunge the toilet or use a plumber's snake to loosen any clogs. Flush the toilet to ensure it will flush properly.
Remove the toilet tank lid. Check the level of the toilet float. Low float settings will mean less water enters the toilet bowl, reducing the flushing action. Raise the float by bending the arm upward or twisting it to increase tank water levels.
Clean water deposit from your toilet. Inspect the tank for any sign of scales in the toilet bowl. Use a recommended cleaner from a home supply store to remove these scales. Flush the cleaner through the toilet to remove build-up in the toilet's plumbing.
Arrange for a plumber to come check the toilet venting. Clogs in toilet vents will result in poor flushing performance. Plumbers will typically have the tools necessary to diagnose and correct this issue.