What is the cause of TCH assign unsuccess ? Radio or Congestion or BSS or BSS Preparation ?
Are you working with Alcatel system (it looks like it) ? In which release ?
Since B9, there is an improvement of the call setup in the concentric cells.
Also, are you sure your parameters are well defined ?
RXLEV UL & DL ZONE = ?
ZONE HYST UL & DL = ?
Do you suspect interference of inner zone ?