Something has to be killing your bacteria, either that or your test kit is bad. Take a sample to your LFS and have them test it. If it comes back with nitrites or nitrates, your test kit is bad.
When you do a water change, are you using tap water and a clean bucket or one that might have been used to wash a car or had some kind of detergent in it? If so, the soap could have killed your bacteria and wouldn't know how long it will take for the detergents to stop affecting the bacteria.
There is still the possibility that it is simply going to take a long time to cycle. I have heard of cases where it took 3 months so 18 days is nothing comparatively.
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