Hi!
It can be any number of factors. First, make sure you have prepared your nib properly as I demonstrated in the tutorial here:
http://theflourishforum.com/forum/index.php?topic=63.0. This can make a difference. Also, check your nib tines and be sure they are even and not bent.
Second, nibs will start to skip and splatter when they are worn out. So if you have been using your nib for some time, it could be time to move on to the next one. I find nibs have a sweet spot, meaning they take a bit to warm up and go smoothly, then they sail across the page, then when they are wearing out they start to skip again.
Third, make sure your paper isn't too porous or textured.
Fourth, vary your pressure to see if you are pressing too hard (or too soft).
Lastly, make sure you are going in the right direction. If you look at the worksheets, there are little arrows and numbers to show the proper direction. The nib will fight back if you are going in the wrong direction.
I hope others will chime in with their tips, too!