Grist composition, grain crush, mash temperature, yeast strain, pitching rate and viability are major players. Do you have a problem with a specific beer? If so, posting your recipe and specifics of your process would be the best way to get help here.
If you're using a refractometer to measure final gravity, you need to use a calculator due to the presence of alcohol. Also, it's important that all of your measuring devices are properly calibrated.