If you want to explicitly enter 16th notes, you have to make it obvious to StaffPad that that's what you want, otherwise it can interpret the lines as 8th or 32nd notes, or a combination of those depending on where the notes fall in the measure. Again, for simple scores where the handwritten music is fairly obvious, it works fine. The key point is that the entry method slows down professional users, because you have to make sure you're doing things that the program can understand, which means taking time to place your marks methodically and reviewing every entry you make to make sure the program was correct to your intentions.
it looks like you'd add dashes for the tails, just like you would for a semiquaver on paper.
one of the great things about this is that there's zero 'design' to sheet music, its essentially a spec.
There's a lot of implied information that can be fed into the handwriting recognition, and it looks like it doesn't try to parse until you've moved onto the next bar, so can analyze the block against a very well defined domain.
Vertical lines all represent a note (besides small ticks for sharps maybe), and the time signature narrows this down to only so many possible permutations of possibilities.
You haven't actually tried out the app have you? To do 8 16th notes, you just need to draw 8 small slanted lines.