if you are getting primer dimer it is probably removing all of your primer before the product is visible Primer dimer is short so does not trap much ETBR so if you can see it at all then you have a lot of it., Try doing a hot start either by using another polymerase or by mixing all the reagents except the polymerase and holding the mix at 70c on the block. then add the polymerase and start denaturation and cycling. At the high temperatures primer dimer should not happen, Include a zero sample control sample in case you already have primer dimer contamination

Dear Banshi,

         I think paul is right. also try with promega taq and increase the amount of cDNA for PCR. 

If the above is not working then check your primer pair once. It is also very important to see the primer sequence details.

Dear Banshi,

1. If you are getting the primer dimer, we can say that amplification is happening but it is not your desired amplicon. So, your reagents are fine for small amplifications as that of your primer dimer. To minimize dimerization you can use a higher temperature if possible. We use 2.5 % of DMSO which works well. You can slightly heat the primers before adding to your master mix to avoid pre-dimerization.  If your template is fine as you have amplified other sequences using it, the only possibility is the primer. As Paul has mentioned about availability of primers for annealing to your desired gene, you can increase the primer concentration up to 1uM and then use a higher annealing temperature to favour the specific base pairing over primer dimer formation. Check the primer sequences if they are properly aligning with your desired segment.

2. Try to amplify another known/ already optimized DNA template of similar length and see if that works. Make sure you are using enough dNTPs and allowing optimal time to synthesize the length of DNA you want to amplify. 

3. Make sure other reagents like MgCl2 is fine and is in optimal concentration in your master mix. The ionic strength of the reaction mix plays important role in efficiency of annealing of your primers to the desired site on the template. Also the KCl concentration in your Polymerase buffer can be altered if your amplicon is very lengthy. You can try to alter it as per your requirement.

Hope it helps!