I'm using NEB's Terminal Transferase (https://www.neb.com/products/m0315-terminal-transferase). I have tried using more enzyme, and tried using more ATP in reactions with no noticeable success. My goal is for 25 adenosine residues so I'm using at least 1:2000 (mol 3' ends:mol ATP). I have tried to identify success on a 2% agarose TAE gel but can't see any shift in product weight (from the non-tailed 128 bp DNA PCR product). The PCR product used in this reaction was purified using a DNA column and eluted in water. As another test to see if the reaction was successful but not noticeable on the gel, I tried to bind-wash-release the product from poly-dT beads and had no success.