I haven’t run into this issue (knock on wood) so here’s a couple thoughts.

1: How are you sending it? Do you put it in a regular mailbox, apartment mailbox, or what? I always drop it in one of the big blue boxes here in the MD with regularly posted pickup hours so I know it’s being picked up somewhat promptly.

2: Are you putting your DPU account number on the actual check? If you aren’t, that could be it. I can’t imagine why anyone would separate the bill slip from the check when they open the envelope, but I once forgot to put my account number on the check when paying property tax in another city and they never processed it.