My tap water will test 1 ppm for ammonia. It is from chloramines.
Different test kits respond differently to ammonia, locked up or not, and different dechlor. There are several types of ammonia tests. The best answer is to use a test kit by the same company that makes the dechlor, and research to find out exactly what the test is showing you. Perhaps an e-mail to the company would clear things up.
When you buy dechlor for chloramines make sure it also locks up ammonia. Some just break the chlorine-ammonia bond that makes up chloramine, but then do nothing about the ammonia.
It is common for the ammonia to show up for a while after refilling the tank. The dechlor has locked it up, but (depending on the test kit) it is still showing on the test.
The nitrifying bacteria should get going on the ammonia, though, and remove it from the tank. 24 hours after the water change you should not still show ammonia.
I would be suspicious of the test kit if it is showing that a cycled tank cannot remove a fraction of a ppm of ammonia overnight. Can you take some water in to be tested, or get another test kit or even a different style of test?