Why don't my muffins rise and develop tops properly?
There are many reasons why muffins might not rise as expected; I'll list the ones I know of in order of probability:


Over-mixing the batter.  You'll know that you've done this if the muffins also turn out tough and chewy.  This prevents rising because the gluten network is too tight to expand around the gas bubbles.
Under-mixing the batter.  You have to develop some gluten, otherwise there's nothing to trap the gas bubbles and the muffins will just deflate before they get any rise.  It's a lot easier to over-mix than under-mix, but if you drastically under-mix and don't even bother to get all the dry ingredients wet, then you won't get any leavening action at all.
Not enough leavening agent. This can happen with unsifted flour (sifting also helps incorporate air), using the wrong type of flour (especially if the recipe calls for self-raising), or using old or improperly-stored flour or baking soda/powder.  You should be able to see some bubbling action before you pop the muffins in the oven; if you don't, you might have this problem.
Improper substitution of baking soda for baking powder.  A lot of people think these are the same, but they aren't.  Both use sodium bicarbonate, which is what produces the CO2 bubbles but needs an acid in order to do it.  Baking soda is intended to be used with mixtures that are already acidic; baking powder has an built-in acidifier, usually cream of tartar, which reacts with the water as soon as you incorporate it.  If you ever substitute baking soda for baking powder, you need to add cream of tartar or some other acid/acidifier.
Not resting the batter, or resting too long.  If you use double-action baking powder (the norm, e.g. Magic brand) then you are supposed to rest for 5-10 minutes to allow for the first action.  If you use single-action baking powder, you must get those muffins into the oven right away or they'll start to collapse.
Finally, and this might be stating the obvious - not filling the tins enough.  It's possible that the muffins are rising just fine, but they're not supposed to double in size like bread or triple like pastries; you should be filling the tins at least 3/4 of the way up if you want tops.