Unless you get a message that indicates the seller is no longer taking bids from you, you may be running into your open transaction limit.
All users, but particularly new users or users that have not been involved in many transactions lately, have limits on how much they can bid at any given time, both the total amount and the number of transactions.
See here for some very general info about limits:
https://www.ebay.com/help/buying/buying-limits/buying-restrictions?id=4012
Any time you place a bid on an auction, that amount counts toward your open transaction limit until the auction ends with another winner or else you win and pay. As you complete more purchases without problems, your limit will be raised. Linking a credit card or bank account to your PayPal account, and linking that to your eBay account can also raise your limit.
To avoid running into your open transaction limit, try bidding the maximum you are willing to spend (taking into account shipping and taxes), once per auction, as late in the auction as you are comfortable with, and then paying for your win as soon as possible before bidding on the next item.
If you add items you are interested in to your Watch List and then sort that list by "ending soonest", and then only bid at the end of the auction, you can avoid needing to bid on multiple auctions at the same time (unless the auctions are ending within a few seconds of each other, or multiple auctions are ending overnight when you cannot bid).
Bidding later in an auction has other benefits, such as not committing you to a possible purchase if a better deal comes along later, and not exposing your bid to other bidders who may bid you up to discover your maximum.
