Coolant problem

Hello,

so we have a Mazak Quick Turn Smart 200M and we filled it up with a semi synthetic coolant. It was okay for a while but all of a sudden the coolant becomes diluted. Oils float to surface, and the % of the concentrate mixture goes down. It's like the concentrate evaporates or something.
We also installed an oil skimmer, but oil keep coming back.

We had no problems with this coolant before, now all of a sudden it has become a problem.

Thanks for the answers!