Dear Ankit,
Understand that you have created a new Posting Term instead using the same posting term. You can use the existing Posting Term also.
Any how in new Posting pls check the partner assigned in your contract as shown in my below screen shot.I hope you have assigned old partner in new posting also, then only chances to get such issue.Pls check the same.
Seems to be you have posted to FI also initially, until what time you have posted only for the 3 months or for long time.I think your issue is lying here, just check this point.After adding partner if you run the Periodic posting ( from the new date)it will surely flow to new BP only if you assigned correctly.
Also you can use same posting term also for differnt BP's, Pls try to refer the below link fully which is very similar to your issue. It may helps you.
Postings : assignment of business partner to contract is limited in time
If still error not resolve pls share the screen shot of the error.
Thanks,
Rao