You might have multiple reason, such as alternative splicing. How do you observe your truncated PCR product (gel, sequencing...) ?

as of now I have obtained as an amplicon of size lesser than expected.

I will have it sequenced shortly.

in accordance to database , the primers were shown to be specific to the target .

so i ended up asking this question

Could be due to deletion within region you are amplifing

Could be alternative splicing, could be deletion or if you try amplify mRNA of gene belong to the conserve gene family you can amplify the other member of the gene family. The best way is send the product to sequencing to check what actually you amplify.

Thank you all for your suggestions