I've been playing violin for four years now and every now and then the bow will get stuck on the strings and not move or produce a sound that doesn't sound right. I don't know whether it's a humidity thing or if I'm the one to blame. Any suggestions welcome. :)
One possible cause (I my opinion the most likely, but I'm no violinist) is, that the rosin applied to the bow accumulated on the strings, so that they have grown sticky over the time. I found a nice recipe for cleaning the strings here. Don't hurry during the procedure, since the alcohol will harm the wood finish.
Several things that could be happening.
As has been mentioned several times above, you may have a surplus of rosin on your bow/strings. Try taking a very soft hanky and gently moving it over the hair of the bow a few times. This removes any extra rosin.
Where on the bow do you play? Play closer to the frog can cause the bow to be sticky n' stuff. Play in front of a mirror or use your selfie cam to record yourself; make sure you're playing more in the middle of the bow (unless a specific technique, like, maybe, marcotto, calls for you to use a little more pressure than normal... Playing near the tip of the bow happens sometimes too in sweeter songs, but really, this stuff is subject to your own technique. Try watching people like Joshua Bell and others to see where they put their bows.)
Too much pressure. It's possible that you are pressing too hard and thus creating that friction that stops bow movement which you describe. Your entire body should be relaxed while playing. You would be surprised at how little pressure it actually takes to depress the strings; similarly, pressing too hard with the bow can cause issues. I have a specific song that's really fast and non-stop and very fast and also extremely fast. At first, I was pressing a little too hard with both my bow and my fingers. Not only was the sound I produced scratchy and not so great, but my arm and fingers actually hurt by the end of the piece. My teacher talked to me about trying to eliminate pressure from my technique, and now I can play twice as fast, twice as well and have twice as fun, all without injuring myself or making weird sounds. After spending several years focusing on building finger and arm strength, being told to lay back on it is a little weird, but you learn to love it. (People complain that the violin is bad for you ergonomically, but I've come to discover that that's only true if you are straining yourself and/or playing too much and too long. Anything can hurt you if you over do it or do it too tensely.) As far as producing a loud enough sound with the bow while still keeping your pressure low, playing closer to the bridge allows for a bigger sound with less pressure. It's possible that you are pressing too hard and thus creating the friction that stops bow movement which you describe.
If you diagnose the problem, give us an update! Happy fiddling!