My 4L60E has missed a 1-2 shift just a couple of times at WOT. It happened fast, so I'm not sure what went on exactly. It either changed 1-2-1-2 rapidly, or the change lasted so long or it went off gear or slipped for a brief time so that rpm limit was reached. Then it did manage to complete the shift. It's also possible, that I managed to let up the pedal a little bit without realizing it, before it upshifted.
I also noticed that the 1-2 shift is a bit softer over the whole rpm- and throttle position spectrum than it used to be. It doesn't matter to the shift firmness, if the selector is on D or OD. It didn't make a difference earlier either.
Other shifts feel good.
My first suspect is leaking accumulator, but I'm a real newbie in this thing... I'm sure there're a thousand other possibilities.
Would anyone more experienced have any advice?