The 99K uses a flat side neeedle and it threads from the left to right. Needle goes in with the flat side to the right. You can't use other type needles, it has to be 130/705 type needle (HAx1 or 15x1), it's the regular for domestic models.
If you aren't used to the type of machine you can in theory mess up threading route of the top thread; it goes from the spool pin, to a tiny hook, down to the two disks in the tensioner, then (!) to the left is a sort of hook with a hole, thread goes through it and up to the lifter arm, to the guide in side plate and needle bar.
Remember to put the bobbin in the correct way from which it is wound, for the thread to stay in place. When everything is set right it should behave.
I'm glad this thread turned up, my 99k very likely needs a fine tuning in the bobbin area.