Here is the clause to insert into your LPA Notice of Rent Increase, Lease Update - Change of Terms Notice or Lease Renewal so that when you issue a notice, it will still be enforceable even if the tenants don't return a signed copy to you.

<!-- Start Copying here --> YOUR CONTINUED OCCUPANCY OF THE PREMISES AFTER ___________________, SHALL CONSTITUTE FULL AGREEMENT WITH ALL OF THE ABOVE IN ADDITION TO AND AS PART OF YOUR LEASE AGREEMENT. (PLEASE KEEP THIS NOTICE WITH YOUR LEASE DOCUMENT) <!-- End Copying here -->

Just copy and paste the above to the bottom of your notice.

Disclaimer: State and local rental contract laws may vary, so The LPA recommends you consult a local attorney to make sure legal notice of this type is enforceable in your local court.