Hi Vu Pham,
If stock exist that should be committed in a Prod. Order. If not got commit then it might be a issue in Material or stock or ATP settings.
Check Material MRP views and Free stock available or not..
Rule control what you have used for the ATP group and Business Event.
If you have used No Checking Horizon, No sub location checkis used then it is ok or try to change it to confirmation at checking horizon.. Its only a Lead time commit..
If all are ok then you need to post more details or others experts need to react if I miss something...
Thanks,
Bala.