Whether you are an artist or an art collector chances are you've come across a canvas painting where the canvas had stretched leaving the painting feeling loose on it's frame. There are a few reasons why a canvas painting can stretch and become loose, but here are the most common reasons.
During the creative process. While the painting becomes wet with water and/or paint, the canvas may become stretched out.
While in storage. If you aren't careful a canvas painting can get stretched out if something gets set on - or against - it.
Canvas quality. Cheaper canvases may come straight our of the package less taut than most artists would prefer.
Regardless of the reason, you can tighten a canvas quite easily with a little water and a hair dryer.
Take your canvas painting and wet the backside of the canvas, Don't drench it, but dampen it lightly. You can even take a wet towel and wet the back of the canvas. (Do not get the front wet.) Then take a hairdryer and on a LOW setting gently dry the canvas from the backside. Do not get it too close to the canvas. This should tighten the cotton canvas as it dries. Repeat if you need to tighten the canvas further.