The water flow through the filter is low.
Check if the lifetime of the filter is finished. If it is finished, replace the filter cartridges.
If the lifetime is not finished yet and the filter is clogged, then there might be too much sediment contaminants depositing inside the filter cartridge. Replace the filter cartridge. You can send an unfiltered water sample to STIEBEL ELTRON, we will check if a POE pre-filter is needed to protect the water filter from clogging early.