Hi Zain,
I'm afraid from here it requires to debug the system in order to find out whether the function module FM_MR_TO_FR_UPDATE is called when you create a material reservations using TA MB21. If you have a developer around ask him to set a breakpoint at the beginning of the form BELEG_BUCHEN with module pool SAPMM07R. Then he/she should try to create a material reservation and wait for the transaction to stop at the breakpoint - continue step by step. There are basically 2 things to watch out for...
- Whether the program flow is actually calling function module FM_MR_TO_FR_UPDATE, and if so
- Why is the module not persisting the funds reservations
Alternatively you may want to consider to open an incident with SAP SE describing that the funds reservations is working in batch (Program RFFMRP_MAT_RESERVATION) but not in real-time using TA MB21.
Cheers,
Danilo