duke-of-bytes wroteConsider their is an air valve on the top
If it's a manual valve that you have to open and close, then make sure you open it when the rain starts so that the tank can fill with water. Otherwise, a closed valve means that the pressure within the tank will start rising as soon as the bottom pipe is filled with water. When the pressure within the tank is greater than atmospheric pressure, the tank will not fill.
If it's a pressure relief valve, most likely it will have a rating tens (if not hundreds) of times greater than atmospheric pressure, which means that it's effectively a permanently closed valve in this scenario, and the tank won't fill beyond a certain point.
Btw, if you haven't done so already, I strongly recommend reading
guidance on use of rainwater tanks as there are many hazards you need to be aware of and know how to address.