There are many possible things that might prevent a bid from being received by eBay or accepted by eBay. Without more details, one can only speculate what might be the particular reason.
Some common issues:
Bidding too late -- the countdown you see on your device is not necessarily precise. In addition, if you wait until the end of the auction to submit a bid and receive an error message, the error may be obscured by the page refreshing at the end of the auction. Try leaving yourself a bit more leeway in bidding to deal with possible issues such as needing to sign in to your account to place a bid if you are having problems getting your bids in.
Bidding too little -- your bid must be at least one increment above the current showing bid; if you are bidding at the end of the auction, other bids will change the amount of the showing bid. If you are trying to bid the minimum amount, any further bids received before your bid is placed will prevent your bid from going through. If possible, calculate the maximum you are willing to spend (taking into consideration shipping and taxes) and bid the appropriate amount once as late in the auction as you are comfortable with. If your bid amount is a round number, consider adding a few cents to avoid the possibility of a tie (which would go to the earlier bid).
Running into bidding limits -- new or infrequent buyers may run into bidding limits imposed by eBay when bidding on multiple items at the same time, or when bidding on particularly expensive items. Some tips on how to raise your bidding limits can be seen here:
https://www.ebay.com/help/buying/buying-limits-restrictions/buying-limits-restrictions?id=4012
You can minimize bidding limit issues in most cases by bidding on one auction at a time and waiting until one auction is either won and paid for or else done and lost before bidding on another auction, except in cases where two auctions are ending within a minute or so of each other or if multiple auctions are ending overnight.
In addition to those issues, there are also many sorts of technical issues that can prevent a timely bid such as various hardware and software problems, network problems, and things like authentication issues -- if you have not signed in to your account in a while, you may need to sign in again to place a bid.
