Combi boiler-pressure low-hot water ok?

Hi, I have a Vaillant Turbomax 242/282e boiler and the pressure has dropped to 0.5. Hot water is fine from all taps and hot and cold water comes out forcefully as per usual. I have looked for leaks but cant find any-though that does not mean there are not any!

In the past I have refilled the system by opening a valve found underneath the main boiler unit (part of the cold water loop, I think), which has immediately increased the pressure gauge. Once it reached below 1.5, about 1.3, I would switch off the valve.

Attempting the same now, the following happened:-
-at first opening the valve, nothing happened-the pressure level stayed at 0.5. Did this quite a few times.
-did it once more, after twisting the stop tap found further down the same pipe as the valve's, and the pressure started to increase as I could hear the sound of water being filled as per usual.

BUT, the pressure took much longer to go up to 1.4, in fact went up at a snail's pace
AND the sound of the water being filled was much quieter than in the past-only reason I knew something was happening as pressure gauge was moving too slow to be informed immediately that it was being refilled.


The question - is this fine or is this a sign of a bigger problem?

Thanks in advance for any help and your time