You already had good information from Thom and Linda and I concur with them. I have written many leases and everything is spelled out in the lease agreement. Also I have never considered the HOA as a utility. Typically the owner pays the HOA dues since it is the owner's responsibility.
How much are your HOA dues? Why not just pay them and let the tenants cover the utilities, simple is always better.
I suggest sitting down with the tenants and start with apologizing for the confusion and saying something like, "let's see if we can come to an agreement we both can live with". This way you can amend the current lease in writing (each party signing the amendment) and hopefully everything will go smoothly from then on.
Brian Wilson, Realtor