with some woods this is caused by oils in the wood.
wiping the wood down with plenty of acetone will remove them in that case
over some stains, such as one color or other of minwax's polyshades, i've had spar urethane fail to dry completely in moderate munidity and low temperatures because the stain wasn't as dry as i'd thought. my solution was to rub the limbs dry with suede, and re-spray in a drier environment. the 3" mailing tubes i use make a good drying tube. i point a hair drier into one end, and seal the seam with a towel, being sure not to cover the drier's intake, and put it on a low setting. this blows dry air across it untill all the volatiles are gone. if that dosen't work, you can be pretty sure [assuming i'm right here] that it's oils.