Why might a dissolved primer work initially but perform poorly after some time?

If the water used to dissolve the primer has a low pH or is contaminated with bacteria or nucleases, the primer can degrade. If the primer is not fully thawed and mixed before use, liquid inhomogeneity might lead to inaccurate primer addition amounts. It is recommended to aliquot primers to avoid repeated freeze-thaw cycles. Using 10 mM Tris pH 7.5 buffer to dissolve primers is recommended, as some distilled water has a relatively low pH (pH 4-5), and primers are unstable under these conditions. Another possibility is that the primer is fine, but the quality of the PCR materials, especially the template, is not identical to that used previously.