
Quoting
Mr. Knowitall
You will have to read your lease to determine what rights you had to the recovery of your share of the deposit at the end of the initial lease term, and if the deposit could be carried over to a new lease even if not all roommates participated in the new lease. If that's what you authorized then the landlord is correct, and you can try to get your share of the deposit from the new roommate while assigning to her your share of the deposit being held by your landlord.
Also, you will have to review the documents from when your new roommate moved in. If you were released from the lease in association with her being added to the lease, you may have agreed in conjunction with that release that your deposit would continue with the leasehold even though you were no longer a tenant.
If neither of the above are supported by the lease or documents relating to the change of tenant, and there is no question or dispute as to your having paid half of the deposit when you leased the premises, then you should be able to recover your share of the deposit from your former landlord and it's up to them to collect the balance from their current tenants.