You've been given some excellent advice by others already, but I just wanted to add a couple of things...
Earth leakage is cumulative. That is, each appliance may have natural leakage (as explained above.) Anything to do with water heating are often guilty. All these small currents add up until a point is reached when the RCD will operate.
On a standard 30milliamp RCD, this is actually around the 23-24 milliamp level. Quite normal. You may have reached this point for one reason or another.
RCDs do not last forever. I have come across this before. In those cases, I ran what is known as a "ramp test." This is a very quick and easy test on a "Megger". It simply ramps up the current slowly until the point is reached where the RCD trips. I have found trips that operated at around 12-15 milliamps. Far below their original design value. Replacing the RCD and re-testing fixed that.
But, before blaming the RCD for over-insensitivity, I would certainly look at the whole installation for possible leakage faults.