Are you sure that the property manager is actually maintaining the property?
If the checks are being returned and they were not returned previously, maybe they are no longer at that location?
If they have not been payed in a while because the checks have not reached, they are not likely taking care of your property.
I am not sure why you were using Cashier checks and not regular checks, regular checks can be deposited in a bank and are just as good.
Getting to the point, I wonder if you are not being taken for a ride, if you are actually getting value for your hard earned money.
If they insist on PayPal, Western Union money transfer or Cashier Checks, it sounds like they are working under the table and may not be doing what you expect. Sorry but I am a little short on Trust when there are conditions like that and or no contracts.
You may want to call friends to check your place out and see if the maintenance company is still at that address.