I had a 70% yankee before buying a new 100% and it was difficult to tack in winds below 10-12 knots. Even now with less than 8 knots of wind I have to assist the headsail between the inner and outer stays.
The key is to make sure you have enough speed (2-3 knots), then hold the headsail until she's past irons (backing the headsail), wait until after the boom comes across the wind, and finally sheet in on the leeward and out on the windward. Don't release the windward sheet, but keep some light tension on it to pass the headsail through. Backing the headsail is an important part of the transition. If it's luffing then there will be nothing to help the bow around.
If you're going slow, fall off a bit and sheet in more to pick up speed before doing your tack. She needs to be moving enough so that she doesn't stall from the turn. I've found the sufficient speed to be about 2 knots. Also make sure you go hard over, making the shortest radius turn as possible. Otherwise you'll slow down enough to stop momentum. Never use an autopilot to make your tack. The pilot arm does not allow for enough distance to push the tiller hard over, which is what you need to do for the tack.