Siemens EQ.9 problem with coffee taste

Dear users, maybe you will be able to give me some advices. Maybe someone from you had the same problem. I am writing you from Poland, where polish service branch office has just ignored me when trying to get some reply.

I purchased my Siemens EQ.9 over a year now (s500 TI955209RW). The coffee machine itself is great for me, great design, functionality and handling ease. The only problem I have generally from the very beginning (but I started to notice this over a few months) is the coffee taste. To be clear: the problem is not with the bad taste, the taste is at least good every time, the problem is rarther that the coffee taste is not as good as it should and as it can be with every cup, even with the same coffee pack. Most the time the taste is just good, regular, but from time to time the taste is just perfect, strong, simply delicious, but only from time to time.

The machine is descaled regularly together with the cleaning program (for both I use original Bosch tablets, I use also the original Brita Intenza filter. I make all this things every about 200 cups (changing filter, cleaning, descaling, also cleaning brewing unit and milk system). The water hardness is low (checked and set on the device). The coffee is always the same (Latte 60% milk, strong - one of the last options in the menu except 2 grindings), the rest of the options are default.

The type of coffee is always the same (Costa Rica Tarazzu) from the same regional small good coffee roaster company. The settings of the grinder are also every time the same.

So please tell me why only sometimes the taste is strong and so good? And most of the times is just "regular"? Not bad but it is much different from the best this machine obviously can prepare. There is no matter when the taste differs (close to the descaling cycle or shortly after).

I wrote to the Siemens in Poland, but after giving them all replies to their questions (which I described above) the just ignored me and didn't write me back.

No errors are displayed, the coffe machine looks like new both inside and outside (I am very pedantic to be honest).
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Thank you in advance for your help.