Tenants having problems with each other - best way to respond?

[Isaak Brown]

Hi there,

I have a four unit building. The top unit has 2 tenants who moved in in April. Below them are 2 tenants who moved in last October. Top Tenants complained last month that Bottom Tenants are putting their trash in her trash bins. Bins are labeled for each unit. I responded that it's her responsibility to speak to the bottom tenants about that.

Top Tenants contacted me about a separate issue today, and reiterated that they're having a problem with Bottom Tenants filling their trash bin and she she isn't comfortable talking to them because they have had another issue with them  about parking (I don't know about that). 

My policy is not to get involved in these disputes. I was clear last time I responded. But I want to make sure I'm on right ground not getting involved. Any advice on how to reply?