Try with the notes shared by Caetano in his blog. I would start with this one:
23922 -
MRP: Creation indicators, PReq. or planned order
To avoid this phenomenon, you should always maintain the purchase requisitions
creation indicator in the planning group for the affected materials or you
should maintain an opening period greater than 0.
If after reading the blog and following all the instructions of the notes, you still have this issue, come back to us.
Kind Regards,
Mariano