Funds are not transfered from PR to PO

Hello Gurus, we are having an issue in our production system which I'm not able to reproduce in our Q environment for further testing.

Sometimes a PR is created, and funds are reserved for that PR, later a PO is created from that PR, but we have no funds!

I checked the movements in FMRP_RFFMEP1AX and I see the original document with the funds assigned to the PR, and then a Reduction for the same amount, and that's it. The PO can't be saved because it lack of budget for that Commitment item/Fund center combination.

We have tried to fix the PR, PO and budget document using FMN3N, FMN4N and FMN5N with no result.

Any ideas why this could be happening and how to fix it/prevent from happening again?.