It could be any number of things. It sounds to me like the gluten isn't well enough developed.
If the same thing is happening in the breadmaker then I suggest that it is one of your ingredients as the breadmaker should give you at the very least a reasonable loaf. Here are a list of possibilities, if you get back to us then we can help narrow it down for you:
-You might be using the wrong type of flour. Use strong white bread flour, I find Allinson's gives the lightest loaves for me. Whatever you do don't use plain flour. There are some other type of flour that will give a heavy structure like rye flour or spelt. It is best to experiment with strong white bread flour as a basic bread.
-Are you using the right type of yeast? you want dried fast action yeast NOT dried active yeast. The latter you have to activate using warm water and is not suitable for the bread machine, also it is a pain in the *** for a few pence cheaper. Yeast is cheap, just make sure you get FAST ACTING yeast.
-Make sure that the salt and yeast don't come into contact as the salt will kill off the yeast and it won't rise much.
-Are you letting the dough rise for long enough? Once kneaded it should be left to double in size (forget timings just look and see how inflated the dough becomes), and when proving (second rise) allow to double again. Proving really ought to take no longer than 1 hr (ish) however as the dough may start to collapse if left too long (especially in a warm place).
-When using the breadmaker, check it periodically. There should be a rest period at the start to allow the flour to absorb the water, followed by about 15 minutes of kneading, then the first rising which is about 1hr 30 min - 2hrs followed by gas release then the proving for about 45 min - 1hr and finally baking (however all breadmakers vary, check your handbook for the exact timings). You can check all the way up to the start of baking to see if your loaf is rising. If it doesn't rise then it is most likely the yeast not doing it's job for one of the above reasons. If it rises and sinks then it may be that the gluten content of the flour is too low and you are using the wrong flour or the gluten in the flour has not been developed enough (although a breadmaker shouldn't have this problem, so check that it kneads the dough properly - at the end of kneading the dough should be smooth, stretchy and holding together in a ball).
I did type out a detailed recipe for a foolproof handmade white tin bread but I didn't have enough space. Let me know if you would like it and I'll delete the above and replace it with the recipe.