There are lots of reasons why a dough won’t rise properly:
Not enough rise time
Not enough gluten – especially a problem if you use whole wheat flour
Not enough sugar – sugar and yeast react together
Not enough salt – salt is needed to limit yeast effectiveness, use too little and it will over-rise
Too much salt – overusing salt will neutralize the yeast
If the bread is flat, I’m guessing that it’s not because you are using too little salt. Try experimenting with the other possibilities, i.e. try adding extra sugar or cutting back on salt. Have patience and you’ll get it right.