You have a C-Max Hybrid in Poland? Thought the hybrid was a US-only model... or do you have a US-spec car?
I suspect if the ambient temps are very high, the battery may avoid high charge levels, as the combination of high ambient temperature and full charge shortens Li=ion battery life. There are cooling fans for the battery; are they running? Perhaps a driving style that features a lot of short burns and glides, so you only use the available charge? Won't work on long hills....