IHS fee for PBS Dependant extension

[Minkigoyal]

Hi
My concern here is about refund of the excess IHS charges paid during PBS dependent application ( which was successful) .
Now almost 4 months have completed after visa got stamped .
But not got refund of the excess IHS charges till now ( these were 2 applications for PBS dependent ).
Bcoz the COS duration of main applicant was 2 years ,so According to that IHS should have been £ 400 . But £600 were calculated by the system for IHS charges for one application .
So £200 were the excess amount charges per application .
This way they should refund £200+£200 for our 2 applications .
If anyone had come across the same problem , please suggest as per your experience .
Your guidance can really help me getting my refund quickly .
I would appreciate your comments .
Thanks a lot .