I am trying to standarize PCR conditions for microsatellites. I include three primers in my PCR reaction, a pigtailed primer, a tagged-primer and a fluorescent labeled primer "the tag" (CAG tag). At the first stage I work with the pigtailed and the tagged-primer each at 0.4 micromolar, I perform MgCl2 titration from 1.5 to 2.5, at this moment I obtain the PCR product of the desired size, the problem arise when I include the fluorescent labeled primer (Usually I include it at 0.4 micromolar and reduce the tagged primer to 0.04 micormolar as suggested by most of the authors), I cannot obtain the product or obtain an unspecified product with the same amplification conditions used for pigtailed-tagged primer pair. I test with the addition of BSA, Betaine, DMSO but nothing works!!!! I also try touchdown PCR protocol but the result is the same

My fluorescent labeled primers are resuspended in TLE (Tris Low EDTA), and are conserved in the dark at
-20 ºC.
I have 14 putative loci, at this moment I have tested 3 primer pairs and only in one I have obtain satisfactory results

Does anyone have experience in the optimization of PCR with tagged primers? I will appreciate any help