Thank you for that information. It is important to point out that a cat that itches/licks excessively in the abdomen area is likely feeling pain within the abdomen. Although pain in the skin can be caused by allergies, other things affecting the skin or deeper inside the abdomen is important to consider. Cats rarely get allergies, so that is a differential lower on the list. Fleas are more common in cats, but it sounds like you have that under control with the application of Revolution. Other things could be mites or a fungal skin infection (i.e. ringworm). Mites are also treated with Revolution, so I doubt it is that either.
More common are things that are painful underneath he skin inside the abdomen. In this case I am talking about intestinal pain or bladder pain, which are both common in cats. Intestinal pain can be due to dietary intolerance / food allergy, inflammatory bowel disease, parasites, foreign body, liver/pancreas inflammation, etc. If there is a history of sporadic episodes of vomiting hair balls then that is supportive of an intestinal disorder. Bladder pain is also something that should be considered because cystitis (i.e. bladder inflammation) is a common condition that can be caused by stress/anxiety, bacteria, or crystals in the urine.
The next thing that should be done is to assess for any possible causes of skin pain that have not been addressed such as the skin mites or fungal organisms, but likely, look deeper at the underlying organs inside the body. The first place to start is with a blood and urine panel. If no causes of inflammation are found then a possible ultrasound of the abdomen, +/- biopsy of the intestines will be needed to rule out intestinal pain. There is no blood marker for the intestines unfortunately, so inflammation in this area needs to be assessed with a biopsy. However, an index of suspicion can be raised based on the history of vomiting hair balls.