are you using the straight box mix as it says on the back of the box? also what size pans are you using and do the sides slope outward at all?
Here are my ideas - could try out one at a time or all at one time...
1. check your pan sizes. I used to have the Wilton 9" pans that came as a set stacked together. They are able to nest perfectly inside one another because the sides are slightly sloped outward. I've read that this may keep cakes from fully rising. I switched to 2 8" Magic Line pans and a mix fills both nicely
2. Check that your oven temperature is accurate with a thermom. inside the oven. I always bake at 325 but have to set my oven to 330 to get there according to the oven therm.
3. Try a cake mix extender to get more batter out of the mix:
4. Add Meringue Powder to the batter to help it rise a little more as suggested above.
5. Mix for exactly as long as it says on the box. 2 minutes usually. I set the kitchen timer.
If none of those help, maybe try a different brand of eggs? Or check the date on the cake mixes? These are just guesses that could affect the rising of the cake?
Good luck and let us know what helps!