Odd group of notes need some help

Very quick question that a guitar teacher may be able to answer or someone who been playing fast for ages

Why don't my fingers enjoy odd grouping of notes for example

lets say I was playing on the D string and went something like this 9 10 and to the G string 8 back to D string 9

I can play pretty good but such things like above causes me no ends of problems any help and advice most welcome. I start slow and learn it but when I try to pick up the pace it all goes bad

Thanks

try and find a position that works best for that situation. for example, you may want to play it like this...
1=index finger, 2=middle finger, 3=ring finger, 4=pinky

  2  3  1  2

E:----------

B:----------

G:------8--

D:9--10---9

A:---------

E:---------