I assume you're referring to oven spring. We need to know more about your recipe and how you're building the bread and baking conditions to zero in on probable solutions.

Many, new to bread making, are surprised by the amount of subtle nuances involved in getting it right. It's not that hard so there's no reason to pack-up right yet although the weather is pretty nice hear.

You've already come to the right place for help.

Wild-Yeast

My guess is the dough is over proofed when it fails to rise during the bake. Deteriming when a loaf  is ready for the oven is, in my opinion, the least scientific thing we do as bread bakers. I know that hasn't helped you in the least, but as the dough rises, poke it once in a while and when it feels like a marshmellow, put it in the oven. After you've made several loaves you'll be able to sense when the dough is ready to bake. 

 Larry

I'd second overproofing as the culprit.  However, another factor might be the way you're shaping your final loaf.  I've found that punching the dough down too much or too little as you're forming your final loaf has a lot to do with the quality of your oven spring.  And I've found that the process by which you shape it (how you form a boule, for instance) has a lot to do with whether it springs consistently throughout the loaf, or whether you end up with lumps.

I've been baking bread for nearly a decade on and off, and I'm only beginning to get the hang of the mystery of oven spring.  My suggestion is that it will come with keen observation of your proofing loaf, and much, much practice. 

Definitely agree with overproofing as a big culprit. Some other possible reasons. No steam in the oven for the first 15 - 20 minutes. The oven not hot enough. The dough way too dry.

Rudy