There are many reasons for blocking a call. Most of them related to interfaces or equipment congestion.
1/ there is no more resource in the cell (but that’s not your case)
2/ there is no resource on the Abis interface
3/ the BSC believes there is no more resource in the cell (algorithm issue, or fake o&m alarm in the cell ?)
4/ the BSC capacity is maxed out : no more calls can be handled by the inner boards.
5/ there is no more resource on the A-interface or on the Ater-mux interface.