^^^
There is an answer explaining the drop you might have missed.
Find feeders nearby and look/compare their performance.
Anyhow, what does your setup build like?
Antenna:
Coax:
Filter: FA filter
LNA: Uputronics
Coax:
Dongle: Airspy mini
RPi: 4B?
If your signal is a bit intermittent, check/replace the coax cable from the antenna.
As @wiedehopf already mentioned: have a little more patience when comparing message rate figures. AND water ingress in coax or LNA is quite often causing these kind of issues.
As I live in the Netherlands, too (Ridderkerk, near Rotterdam) it might be relevant to compare your performance fluctuations with mine.
On october 10, I replaced my antenna, filter and LNA.
Send me a PM and I’ll give you a live link to my feeder. If you live in the vicinity of Rotterdam you can compare our feeders live (i.e. with MM2 and Tar1090) by opening two browsers beside each other.
I can confirm that (been there last sunday 
)