Just did a quick google search and found this
I don't have enough time to read and explain it here in short because I need to go somewhere but here have a read.
http://www.madsci.org/posts/archives/2000-05/959607056.Ph.r.htmledit: last few lines
Remember that a droplet has surface tension, and attracts the sides of the straw. The only way that air can get into the straw is by BREAKING the droplet's surface tension. If the straw is narrow, air is not strong enough! Sometimes, for a medium-diameter straw, if you tilt the straw, the weight of the droplet will pull itself away from part of the straw. This will help the air break the surface tension, so it can get in. The water will drain out, drop by drop.