Need advice on really long fishless cycle. The problem - lack of nitrates

Hi delta678, welcome to the forum!  

First, congratulation for doing a fishless cycling.  You are on the right track to become a responsible fish keeper. 

Second, I have looked at your problem.
In order to find what's causing the issue, I need to ask a few questions.  

1.  Have you checked the ammonia concentration with the test kit to make sure it was 2.5ppm?
Adding ammonia by calculation is good, but we still need to test it afterward to make sure the concentration was up to our expectation.  

2.  What kind of water test kit are you using?
If all your ammonia has been turned to nitrite on repeat basis, while nitrite reading is not going up over the weeks, then it is very likely you indeed have nitrate but you are not getting an accurate reading for some reason.

If you are using API freshwater master kit, the nitrate test is a little tricky.   It has been a while since I used this water kit, so I do not remember exactly procedure.    If I remember correctly, you need to add two different types of chemical compound into the test tube.   The one thing I remember is, you have to shake the test liquid for up to a minute before and after the mixture.  Otherwise the nitrate reading you get would be 0ppm.   (additional information: Finally remembered it, you need to shake one of the test liquid for 30 seconds to a minute before you even open the bottle)

Some other water test kits might also have the same tricky procedure.   Make sure you follow the instruction exactly to get an accurate reading.  It is very important for fishless cycling to get all the readings correctly.