Low CSSR due to poor assignment success…
First, which assignment ? The SDCCH or the TCH ?
Possible low CSSR (depending on your CSSR formula !!)
poor radio conditions on BCCH
poor radio conditions on SDCCH
poor radio conditions on TCH (unable to access the ts TCH)
and hardware failure : abis, combiner, tre, crossed tre, crossed sectors, etc…
regarding IMMEDIATE ASSIGNMENT, the possible failure cause is mostly SDCCH congestion. Check the SDCCH traffic and resources usage, you might notice you’re exceeding the offered traffic.