Like my previous experience in Dr. NT-Lin lab, The negative staining solution 2% UA solution is too old. If you have a resource in your lab, prepared the new UA solution. Contrary, It also depends on your phage titer still in the same value or not. As my virology knowledge, if you keep your phage long time in 4-degree condition with LB medium, it has the possibility to reduce your phage titer. Even tere are so many things you need to consider carefully when you perform TEM experiment.