SLA dates are not calculated when incident is created

Hi Experts,

I have configured SLA based on multi-level categorization schema: level 2 is Severity level 1,2,3,4 and for each severity I created a service product with specific response times.

It works only partially, i.e. when an incident first created and I select Severity in Level 2 category, and save the incident for the first time, what happened is : it figures the response profile correctly based on severity - I can see it in SLA assignment block - however dates are not calculated and left blank.

Once I make a change to Priority field or Level 2 (severity) - and save incident again - the dates are calculated properly.

Why they are not calculated at the ticket creation time ? What can be a problem ?

Appreciate any help.

Thanks

Elena