Hi Georgi,
In standard SAP the availability check will not happen for STOs which are created automatically , as described in the below link
http://help.sap.com/saphelp_erp60_sp/helpdata/en/43/543835bc0f357ee10000000a11466f/content.htm
I would suggest you run any kind of Rescheduling of STOs to check the availability for STOs using Backorder processing in SCM or V_V2 in ECC after TLB run.
Regards,
Kiru