I have a portfolio of Section 8 rentals. Here's what I (or maybe in your case, your PM, up to you) would do.
Contact the tenant to inquire about happened.
If their income has changed, they can & should contact their caseworker to submit new income docs. Section 8 may (in my experience, they almost always will) change their rent portion. They might even back date it depending on when the income change happened.
If it didn't change, let the tenant know that my process is to file for eviction when 30 days late. Then I'd say - I don't want to do that and I don't want to put your voucher at risk (if they get evicted and as long as you notify the caseworker/Housing Authority, they should lose their voucher until you're paid). So what's your plan to pay? I suspect this will get them to pay.
As long as they have a plan, it's short time-wise and they stick to it, I wouldn't notify the caseworker.
I would enforce whatever late fees are in my lease.
If they say they can't or won't pay, send a statement by email to tenant and caseworker ASAP and let them know I plan to send to the attorney for eviction on XXX date. I would request a response/receipt from caseworker and would email or call daily until I get one (politely, of course).
As for utilities, notify the tenant that these utilities should have been in their name from day 1, and I will be billing the utility charges to them (as long as that's what the lease says). Add the utilities to the tenant ledger, send copies of the statements to the tenants and caseworker.
Finally, if a tenant leaves a subsidized unit still owing $ to the landlord, they shouldn't be able to move elsewhere until they've paid the landlord in full. But this requires communication to the caseworker/Housing Authority on the landlords part.
I find good Section 8 tenants pay their portion religiously because they value that voucher. Almost all of my tenants pay a portion of their rent and I don't have trouble collecting. But I make clear from move in that the lease will be enforced, including payment policy, late fees and evictions.