Why does my boiler constantly need topping up?

My boiler is about 7 years old, so not too old. But a few times in the last year, we've had to call out a plumber because the water pressure keeps dropping and it trips something inside. We have to top it up every 3 weeks or so - is that normal??? And what could be causing it? There's no noticeable sign of a leak anywhere. Is a leak the only reason the pressure could be dropping so quickly? Thanks!