Many times there is a delay between events occurring and the MLS database's being updated to reflect that event. If an offer was executed in the evening, it is common that the executed contract is not delivered back to the offering party until the next day.
The listing agent has then a few days to update the status of the listing. Don't be surprised if the second day after execution the status is still not updated. It is not a violation of MLS rules until 5 days have elapsed from the event to updating. If the weekend falls during that time, it can stretch even longer.
Can you still make an offer on a property that is already under contract?
Yes, of course. The listing agent is required by TRELA to present all offers in a timely fashion to the seller. The Realtor Code of Ethics also requires all offers to be presented to the seller for consideration (and usually the agent is required to give advice on what to do about the offer). Failure to present the offer is a violation of law and ethics, and subjects the agent to significant penalties.
Does the seller have to agree or even to entertain the offer?
No. The seller can ignore, reject or counter the offer, besides accepting it. However, if a contract already exists, then the offer must be made contingent on the existing contract's being terminated. We call the second accepted offer contingent on the first a "backup" contract, and a form is included in the offer describing the fact that the contract, if executed, requires the first contract to terminate.
The buyer's agent should know how to include the "backup" addendum in the offer package to the seller who has an existing contract.
Be warned, though, that on short sales listing agents often confuse their duties with the desires of the seller's bank. The bank may ask that all offers be shown to the bank or it may indicate that the deal is approved and no more offers will be entertained. Neither of these is actually completely true. The bank, unless it has a separate agreement with the seller/borrower, has no right to see all offers.
The seller is free to withhold any offer he likes, but if as part of his arrangement with the bank, he already agreed to show every offer, then that would obligate him to do so.
Whether the bank believes the offer is acceptable is not relevant to whether the seller signs a backup contract. The seller can withhold the backup contract from the bank unless/until the first contract fails, if the bank does not want to be bothered by other offers after it has approved one. But, the bank is not stupid, and listing agents who withhold a higher backup offer may find themselves at significant risk of liability for not notifying the bank that a higher offer has been made, even if the first one is approved. The reason is simple: money. A higher offer may result in the bank's losing less money - the broker may find himself coughing up the difference in offers if he hides the backup.
Feel free to ask your agent to go ahead and prepare a backup offer. If she looks confused or refuses to do as you ask, find a new agent to help you.