Garbage Disposal water not draining

[Jennliner]

I recently had a new garbage disposal installed in my single bowl kitchen sink. I also have a dishwasher that is attached to it.

1. The dishwasher water backs up into the sink 3-4 different times during a regular cycle. When I see the sink is full, I turn on the disposal and the water goes down the drain.

2. When I run water from the faucet into the sink, it goes down the drain as normal for a while, but always eventually begins to back up and fill the sink. Again, when I turn on the disposal and the water goes down the drain.

Anyone have any ideas on what to do to fix this?
I have checked for clogs in all pipes to everything under the sink
I have check for blockages on the vent pipe
I have used enzymatic drain cleans
I have used root destroyer through the toilet

I read that the rubber baffle may have a tight tolerence and can be trimmed with a razor blade. What is a rubber baffle? I have searched diagrams of disposals and have not been able to locate this. Is it called something else? Does anyone have a picture of one and/or very detailed directions on how to do this procedure?

[LazyPup]

I see one possiblity that is commonly overlooked. When the new dispoasl was installed did they by chance replace the P-trap? If so, check the length of the P-trap where it enters the trap adapter to the waste arm ( where the p-trap enters the pipe coming out of the wall) Quite often the P-trap is a bit too long and goes too deep into the waste arm. The end of the trap is then into the radius of the 90 deg fitting on the waste arm and causing an obstruction to flow. This is common in older homes that have metal pipes in the wall. The new type trap adapters used on PVC have a stop in them so the P-trap cannot go too deep into the line. The P-trap should properly extend about 1/2 to 3/4 of an inch in beyond the compression fitting on the end of the waste arm.