Jodi
When this happens I reach to them via certified mail and first class mail (send both) telling them one of the following:
I underhand how life can take over and cause you (the tenant) to not find the time to contact me regarding the lease situation we discussed. While I do not want to lose you as a tenant I understand that you have choices to make, as do I. Because we have been unable to discuss this matter further, I have enclosed a the required Notice of Rental Increase to $ ($ + 4%) {or if you want to terminate the tenancy , a Notice of Termination}.
This usually gets their attention and they quickly call. If they do not call then you are going to receive a higher amount you wanted.
Either way these tenants are thinking of terminating the tenancy so why not "assist" them?
What is key for you is that you are getting what is fair on your terms.
You can be fair and nice at the same time - trust me they may not think of it that way but you know you are right.
Hope this helps...
Steve