You say you live in a tropical climate. It is probably humidity that is doing it. Do you keep the film refrigerated? Is the film in the camera a long time and in and out of AC? I'm assuming this is 120 film.
That is more than likely the problem. I don't have a solution for you besides avoiding changes in temperature which in a humid environment causes condensation. Make sure film is at ambient temperature before you break the seal. You could also try presoaking the film before development. I've had the same thing happen with different films so it isn't the HP5.
You don't need to develop film at 20º either. Just try to keep the developer under 25º or so. There are temperature conversion charts to give you the right time.