Why did my electroporation suddenly fail?

Recently, I have got a problem in my electroporation. Previously, performing electroporation had been successful. But suddenly, since several months ago, all experiments failed and no clones grown after electroporation (<10000 clones per pulse). I examined all of factors that may affect electroporation efficiency: new cuvettes (1mm,2mm, from biorad), two bio-rad electroporation system (Gene Pulser Xcell™ ;Bio-Rad Gene Pulser II Electroporation System); 2 positive control plasmids (working in chemical transformation); electroporation buffer (freshly made 10-20% glycerol, which does not degrade DNA after incubation with plasmid); competent cell (freshly made TG1 and stbl3, verified by a normal doubling time and minimal nutrition plate); washing times (3-6 times with pure milliQ water or 10% glycerol, on ice, viability is fine even after freeze & thaw); 2YT medium and SOC medium( reagents from 2 laboratories); electroporation parameters(1.5-2kv, 25uF), recovery time (1h-2h). Only one different phenomenon observed is that previously successful electroporation has a various pulsing time (4.7-5.2ms), but now the electroporation pulsing time is quite constant (5.2ms-5.3ms). This is quite weird. Electroporation experts, could you please advise me on my electroporation? Thanks.