When signed in, if you look at the item that you bid on, it will either say You are Losing or You are Winning--hopefully the latter.
If there is no red or green notification on the item, then your bid was not accepted.
Another way to see the items you successfully bid on: Sign in and go to My Account in the upper left. On your Dashboard, you will find a table with a list of the items you have bid on. You'll find the You are Losing or You are Winning notifications here, too.
If you have placed a bid and it is not registering, the reason is most likely because some required fields were missed during the bidding process. When trying to bid again, please be sure that you DO NOT see This is a required field in red below any form fields. If no form fields have been missed, then click on Place Bid and your bid will be registered.