What is the temp of the tank, low temp will prolong the cycle. Extremely high ammonia can inhibit the bacteria, that is another possibility. If your tank is extremely clean to start with (perhaps you bleach or sterilized everything?), there won't be much bacterial spores to start your cycle. <It will eventually get there in the air, but might take a while, esp. with an enclosed tank.> Finally, did you dechlorinate your water with your pwc? .... chlorine/chloramines will kill the filter bugs as well.
As to why you can't bring the level down with water change .... How much are you changing? With a level of 8, you need to do a 90% change just to to get down to 1 ... <And 8 is at the upper end of most kits, the actual levels may even be higher!>
First thing I would do is to test the tap water, just to make sure there is no ammonia. Next, you will need to do some big water changes. I would also do a gravel vac to get rid of some decaying stuff in the gravel, and to feed the fish less so there is less waste produced. Next, I would try seeding the tank. If you can get a hold of a healthy tank (or pond), get some of the filter gunk (or a bit of the filter) & put it into yours. <Failing that, there are posts on the net suggesting getting a bit of clean garden dirt as your seeding source .... depending on what your have in your garden .... may or maynot be advisible .... definitely do this only if you are 100% organic - no pesticides/herbicides/lots of ferts(=ammonia).>