Two thoughts...
1. Neilanh's suggestion comes from the need (as I understand it) for a little bit of phosphorus to get the 2nd bacteria established that converts nitrite to nitrate. Most fish foods are somewhere between 1% and 10% phosphorus (read the can, it should say something about phpsphorus levels).
[Edit... the comments below were based on faulty recolection of test readings. I've given number I was getting for nitrate and reporting them below as nitrite. In concept, everything I've said below is logically correct, but I've got my reported nitrite readings off by about a factor of 10.]
2. The 1st bacteria (ammonia to nitrite) is easy to establish, but the 2nd (nitrite to nitrate) is a little more difficult. While you are waiting for the 2nd bacteria to become established, the 1st bacteria will be converting all the ammonia you're dumping in the tank into nitrite with almost nothing there to convert the nitrite to nitrate. At this time, your nitrite levels can easilly go through the roof. I know in my case, I at one time had a nitrite level of 150+ppm. At that level, two 75% PWC will still leave nitrite levels at 10ppm. Even a third 75% PWC will only bring it down to 2.5ppm, and that assumes no additional nitrite is produced during the PWCs.
It's even possible for your cycling to be complete and you just don't know it yet.
Think of it this way...
Let's say your fish are going to release enough ammonia per day to raise the ammonia levels to say 3ppm with no filtration. For your tank to be cycled, you need enough bacteria build up that they can convert 3ppm of ammonia to 3ppm of nitrite in 24 hours and 3ppm of nitrite to nitrate in 24 hours. If the ammonia==>nitrite bacteria got so far ahead of the the nitrite==>nitrate bacteria that your nitrite levels went to 120ppm, even if you now have enough bacteria to convert 3ppm of nitrite to nitate, it will take 40 days for the bacteria to do so (assuming it did not continue to multiply).
So if you are doing a fishless cycle and you think your cycle has stalled, here is what I suggest:
1. Do as many PWCs as needed to get nitrite levels BELOW 5ppm. Remember, at 75% PWCs, this could easily require 3 PWC to get a huge spike in nitrite levels down below 5ppm (at it's max, my nitrite levels exceeded 150ppm).
2. Continue to add ammonia at the rate you were adding before (i.e. what ever was needed to say raise ammonia to 3ppm).
3. After 24 hours, test nitrite levels again. If the nitrite levels continue to rise, then you are not finished cycling, and in a few days your nitrite levels might spike again such that PWC will be required to bring it back down below 5ppm. If the nitrite levels are the same or dropped, then your cycle is nearly complete. If nitrite levels are zero, again dose enough ammonia to get a 3ppm reading, wait 24 hours, and test nitrite levels again. If they are still zero, then your cycle is complete (at least to handle a bio-load equal to the amount of ammonia you were dousing the tank with).
This is exactly what happened to my first fishless cycle... my cycle was actually finished, but because I had had such a HUGE nitrite level, I just didn't know the cycle was complete.
