In the order they actually happen:
1. The item already closed
The most common one. Bidding ended a second before you tapped. Nothing is wrong; you were late.
2. You are already the highest bidder
You cannot bid against yourself. The control waits until somebody outbids you.
3. Your bid was below the current step
The price moves in increments, and they rise with the value. A bid below the next step is not a valid bid. What increments are.
4. Your connection dropped
Usually brief. Reload the show; your existing bids stand. If the video is also stuttering, that is the same cause.
5. An unpaid win is blocking you
If you won something earlier and did not pay, bidding can be blocked until it is cleared. What that means.
If none of those fit
Note the item, the show and roughly the time, and tell us. A bid that should have landed and did not is a bug and we want it.
Questions people ask
How do I know whether my bid registered?
It appears in the bid list with everyone else's. If it is not there, it did not land.
Why did the price go up by more than my bid?
Somebody bid above the minimum step rather than at it. People do that to end an argument rather than crawl up one increment at a time.
Do I need an account to bid?
Yes — watching is open to anyone, bidding is not. Creating one.
More: how bidding works.