A boiler may keep losing pressure due to a few common reasons. One possibility is a leak in the system, which could be from the boiler, pipes, or radiators. Even a small leak can cause a noticeable drop in pressure over time. Another cause could be a faulty pressure relief valve, which may be allowing pressure to escape. Additionally, bleeding radiators or recent repairs might also cause a temporary loss of boiler pressure. Regular checks and maintenance can help identify the exact issue.