Actually, there are many reasons for high TCH blocking rate,for example, in ur case ,i think it is lack of TCH resources, I mean you may have a cell with small noumber of TCH(small noumber of TRXS)& this cell may be located in highly populated area,So ,the TCH of this cell being always bussy and this certenly cause high THC blocking rate.
Another reason for high TCH blocking rate is that your antenna sector is rongly directed and suffer over shotting ,it also cause lack of TCH resources.
Another reason for TCH blocking rate is HO, you have to check ur HO parameters setting(outgoing & incoming HO threshold.
IF you find no proplem in above points, then you have to check your STATISTIC TASK in UR KPI report.
If any body can tell us more or give his openion ,please let’s share knowledg.