I've had issues on occasion, but usually its normal.

The times that seem to be consistent:

- I have never ever received the regular confirmation email (your purchase has been confirmed) when booking through the UA app (never an issue when booked from the regular website).
- The email receipt function shows up but doesn't work on the app if you haven't been ticketed yet - really - they should hide that option if its not supposed to be available like they do on the website. Examples are just after purchase but before the ticket has been issued, or if you have a held (Farelocked) reservation

I've had occasions where the email receipt link (on the regular site or app) hasn't worked, but usually, just wait a bit, then try again and it works.

It is Friday evening, when UA typically starts their weekend update (schedule changes, etc.), so perhaps that is affecting that delivery or a pending change on your flights. If this is the case, I'd think it should work again by mid-late morning.