Hi Mohamed,
SLA will trigger automatically only if it is a default SLA. And by system only one SLA Can be Default.
What you need to do is create a single SLA and have your conditions as SLA Items and set that SLA as Default so it triggers for all the cases and Appropriate SLA Items are triggered for each scenario.
With current configuration as your, you need to manually apply the SLA to the items which are Critical cases.
So I suggest to create SLA Items for each configuration you have and have the condition for each items properly configured.
If my response answered your question, please "mark the response as an answer" and also "vote as helpful". Regards, Hari. www.crm2011byhari.blogspot.com