What to you do if the tenant will not prove renters ins with the landlord listed as the addl insured? All that I know to do at this point with the stragglers is Send a lease violation letter and raise the rent $100/month until proof is received.
Is your lease with the tenant month to month, or is it a "term" lease, ending sometime later this year or next year? You can probably send her/him a notice of a lease violation, but you cannot raise the rent until the end of the lease. Regardless of when the lease ends, if you give proper notice to raise the rent, please remember, the T can also give you notice to move.
If you label the $100 as a result of the T Ins violation then T can sue you for retaliation etc.. so under any circumstance do not justify your increase because of the T failures.
You may keep your own records for the reason of the increase.
or instead offer a discount for proof of Insurance... so at a later date increase the $100 on its own. then after that offer a discount of $110 for proof of ins. this is an example of an unrelated charge/discount