Tenant keeps getting mold in bathroom.

[Alexander Rodriguez]

I bought a triplex over a year ago. One of the tenants, who seems to be clean, the times I have entered their property, had a window trim in her bathroom that was full of mold and kept spreading when she cleaned it. The window trim could not be fully cleaned, so I replaced the window trim and made sure the bathroom was very clean. I fixed the exhaust fan which had no connection to the outside, and her window works fine. Now one year later at an annual inspection the mold is almost the way it was in the beginning! I thought I had done everything I could to fix the issue. To start off, I think the tenant should have let me know sooner before it got this bad, am I wrong? And is this an issue I should fix? Is there anything that is my responsibility that can cause this? Or is this a tenant issue? Please help!