Rt-pcr primer issues?

I am using a Bio-rad MyiQ rt-pcr. I am screening some bacteria strain for 2 genes. I get lyophilized powders of the primers and make a 100um stock and from that make a 2.5 um stock. I freeze all the primer stocks in the freezer and defrost on ice before use. I add 7.5ul of dna into the pcr well, 2.5ul of both the forward and reverse primer, and 12.5ul of sybr green supermix. One of the gene has been amplifying fine with no problem. The problem is that the other gene has been amplifying at first and having a melt temp of 82 and all of a sudden the primer stops working. It now just gives me a melt temp of 78, which is just non specific binding. The other aliquots of the primer doesn't seem to work with me making a new 2.5um stock. I have been defrosting and freezing the primers only twice, but sometimes I have been defrosting the sybr green multiple times. Is there any problem that I did when I originally made the 100um stock and froze it right away? Should I defrost the primers on ice all the way, so they are completely melted? Should I change the amount I put into my pcr plate or change my pcr protocol? This primer works fine when I first get it and then it always starts to not function as time goes by.