Have never tried M. nigra, but graft most of my mulberries in April, dormant-collected scions, after rootstock breaks dormancy, using a simple bark graft. Generally get successful unions >90% of the time.

Thanks lucky_p, either I'm doing something wrong or Morus nigra is more difficult to graft, or both!

Would you be willing to type out all the bloody details, which is to say, "How many clues will you provide for other grafters to look for areas to suggest improvement?" Who, what, when, where, and why stuff. Trying to 'name that tune' with only a couple notes heard is pretty tough......Personally, I choose to keep notes on grafts done to keep track of things done right, wrong, or iffy. An example of the 'iffy' was a month ago when the only Leona pear scions that I had were already starting to swell big time and showing yellow while still in the fridge. Normally I would not bother trying to graft them, but this choice variety was not easy to come up with, so I gave it a shot. My notes show that I soaked the freshly cut bottom 1/2" in Super Thrive nutrient mixed in water for a day before doing bark grafts with Parafilm stretched pretty thin over the puffed up buds. Two died, one struggled growing baby leaves out before withering away, 2 are slowly growing out from a single bud on each, and 2 are growing crazy fast. Notes work much better than my declining number of challenged brain cells, especially when trying to remember over long periods of time...like 2 weeks....

Mulberry do not graft well when dormant I have tried. The trick may be to find compatible wood once the timing is right. Graft once the trees are breaking dormancy and the sap is flowing. They are not like an apple or pear you can graft in March. Graft them more like a peach or cherry.

I'm not sure. Can't recall if I've ever done simple whip or W&T grafts of mulberry... seems like maybe I tried a few early on, with minimal success...and switched over to bark grafts.
If excessive bleeding seems to be your problem, try what I do with most pecan/walnut grafts - the 'sap-stopper' modification - instead of completely beheading the rootstock, just cut about 3/4 of the way through and break the top over, leaving it attached as something of a 'pressure relief valve'; then do your bark graft, and cut the hanging top off once the scion begins pushing buds.
Or, as I was instructed to do with 'banana' grafts - make a couple of near-complete circumferential cuts in the bark below the graft (don't remove bark, just a couple of quick slices) - allows for some leakage, and helps concentrate auxins above the 'girdling' cuts

Thanks lucky! I guess it won't hurt to try that next year… I'm also going to try some cuttings and air layering this summer.