Dill's right. It would be helpful if posted your recipe.
But assuming it's basic French bread: flour, water, yeast and salt in (more or less) the usual proportions, the recipe itself probably isn't the problem. More likely it's your technique.
Also, as you're getting two good rises the yeast itself probably isn't the problem.
What is the problem is that your loaves don't have enough air in them.
A lot of bakers who are used to making American type breads which include sugar, milk and other yeast junk food in the recipe; and also use loaf pans for formation have trouble making the switch to the gentler handling French and Italian breads and free form loaves in general require.
It's generally best not to "punch" the risen dough down; rather remove it from the rising bowl, "pull it down" gently, fold it three by three, and put it back in the rising bowl (or the banneton). The idea is to keep the cells which formed during the rise as intact as possible; and retain as much as air as possible in the sponge while (paradoxically and) simultaneously collapsing it.
When the air inside the cells heats and expands, the cells themselves expand. In turn, that creates the open, airy crumb structure of good French and Italian bread.
The last rise (whether it's the second or preferably the third) should not be a full doubling. Rather it should go two-thirds or three-quarters of the way to allow the bread some oven spring. Too much rise on the last rise before baking is often a factor in a loaf collapsing in the oven.
In addition to what's going on inside the bread, it's likely you have a problem with the outside. Your bread is almost certainly collapsing once it gets into the oven because there's not enough "surface tension" on the outer skin to hold the shape. Once gravity pulls the loaf down, neither the yeast nor the cell structure created during the rise is enough to make the loaf rise again. The tension is created when the dough is pulled down and the technique used to form the loaves (unless you're using a banneton or pan form).
Pulling down is basic. You hold the ball of dough in one hand and use the other to grab some dough from the top and pull it down towards the bottom of the ball. Then turn the ball a quarter of a turn and repeat. And repeat. And repeat a few more times. Just keep on doing it until the skin is stretched tight. The trick is to do it gently enough so as not to destroy the leavened structure created by the yeast. Don't worry -- you get better as you go along.
Of course, a lot of what I've written to you is based on my hunches as to the actual nature of your problems. At least they're informed hunches. If anyone knows bread baking mistakes, it's me. Heaven knows I've made them all.
Write back and let us know more exactly what happened -- not only the recipe but how you handled the techniques. We can break it down more competently, and go from there.
Hope this helps,