@Laxmi, first let me mention that there is an issue associated that has been reported to Atlassian where the SLA does not start when creating and assigning an issue at the same time. Having said that, let's not simply assume that is the issue you are experiencing w/o answering a few questions:
If the answer to the second question is yes then suggest you test w/o assigning the user when creating and see what happens.
If the answer is no to the above then could I request you provide some actual screen shots showing your SLA and one issue where the SLA are shown including creation time, etc.
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