2. Collimation, (optical alignment) could be the problem. Visit this site on how to fix this and many other telescope problems. http://astronomy.trilobytes.com.au/scope/fix-it.htm

3. Bad seeing (turbulence). If Jupiter is low in the sky, atmospheric turbulence can obliterate contrast and detail on the planet. Wait for it to gain altitude say greater than 30 degrees. Sometimes seeing may be bad even when Jupiter is high. It depends on atmospheric conditions at the time.

4. Speaking of turbulence. Thermal currents inside the telescope tube will have a similar effect if the scope is transported from a warm interior to cold outdoors. Allow the scope 1/2hr to 1hr to reach thermal equilibrium outdoors before beginning to search for planetary detail.

5. Even in an 8" with good optics and good seeing conditions the main features of the planet require some patience and practice to discern especially if you are new to observing and have been spoiled by wonderful Hubble and interplanetary spacecraft images . With a 3.5" (90mm) Jupiter's two equatorial cloud belts plus one or two fainter ones should be visible with some internal detail. With good seeing the Red Spot may be detected as an "indentation" in the south equatorial belt.

6. Sometimes a larger aperture performs worse than a smaller aperture under bad seeing conditions. You may have been caught out by this. It depends on the prevailing general seeing conditions in your locality which may be determined by season and the thermal effects of adjacent buildings, asphalt, bodies of water, vegetation and other such factors. You might like to try a 4" or 6" aperture mask over the top end of the scope to see if this beats the seeing conditions. It might also cure a badly figured homemade mirror if that is the problem.

7. Are you pushing the magnification too high? Telescopes have a magnification limited based on aperture size. It's the laws of optics! Beyond this the image degrades in quality. If seeing is bad it only compounds the degradation. Use moderately high powers like 40-50x per inch of aperture. E.g. for an 8" scope the formula gives 8x40=320x. The simulations above are based on 40x per inch.

8. Sometimes a blue or green  filter will help if the seeing is good to begin with and your scope aperture is 8" or larger. Otherwise it makes little difference. 

9. Provided the optics in your scope are good to begin with, keep on persisting with the above points in mind and you are sure to get a good night when Jupiter will deliver! As a rule, with good optics, under good seeing conditions, a larger aperture will always outperform a smaller one.