It's difficult to give an answer to your problem, really. I'm struggling with midi drum import myself, not with SO though (Logic, Cubase). Sometimes it works great, sometimes the result is a real mess. An obvious task is to check whether or not your drum part in SO is set on the proper channel, that is channel 10, as a forum member once pointed out. Open the file in Notion and choose convert to notation in the tool menu. Then hold command T (on mac) and allocate drum kit. If not on channel 10, Notion will understand your drum part is not a drum part, but rather a piano part or any other instrument and you'll end up with a treble and bass clef staves, with ledger lines and notes placed not where they're supposed to be...

Checking this channel 10 issue was a time saver for me BUT I must say it doesn't work all the time, and I can't figure out why.

You'll find many discussions about midi drum import to Notion in the forum. Things are improving a lot, but it is difficult to know why you just don't get the same results all the time. And there is no drum mapping in Notion. It is a futur feature request, but I guess it'll take time before it is implemented in next Notion releases.