I have 36 on each needle but they keep falling off and I don’t know how to pick them up, so I undo everything and start over. This is my third try and it is so frustrating. What am I doing wrong? My needles are 7". Should I
buy longer ones?
You might want to consider a circular needle for that many stitches.
Definitely try a 16" circular for that!
Once you decrease to a decent amount to use on DPNs, if your needles are still loose, try putting rubber bands around the ends of the needles. (This also works if you’re using large straight single-pointed needles and your project is slipping off the back of them - wrap a rubber band a few times around the metal button.)
Thank you both for your good suggestions. I ripped it all out and started again on circulars and am much happier now. Only thing is, I was so pleased to see the hat develop that I stayed up WAY past my bedtime and now I feel it ! How do I determine what a “decent amount of stitches” is to put on double pointeds?
You can have more stitches on wooden DPNs (in my opinion) because the stitches don’t have a tendency to fall off as easily.
When it gets too small in diameter to work comfortably on the circs.
sue
I only use dpns if it’s impossible to use circs for the project.
But don’t get too crazy avoiding the DPNs. Someone on here (I don’t remember who or I would TOTALLY call her out) kept going so long that the work was too tight and she couldn’t get it to move at all so she couldn’t get it on the DPNs. Besides doing that ends up stretching out your work and ya don’t want that ya know! 
I think the basic premise here is that if you have so many for dpns that they fall off, you might as well use a circ and go back to dpns when it’s reasonable.