It is all in the seller's instructions. If they have accepted the first offer they received there is little you can do. Possibly you could submit a back up offer but the seller often directs no back ups. Many times they will agree and it takes time for the assest manager to sign. Many agents will note in the remarks to the Realtor something like "accepted offer" Have your agent call the listing agent!!! I do not believe you side them.
If you feel the agent is acting inapproiately call their Broker or the Realtor's Board in your area.