There are actually quite a few different factors that can affect how much bread rises (or doesn't rise, as the case may be) in a bread machine. Here are just a few that you may want to consider:
- The temperature of the water or milk. Whichever liquid you use in your bread recipe needs to be warm, but not hot. If it is too cold, the yeast won't be properly activated. If it is too hot, it will kill the yeast. Ideally you should shoot for a temperature of around 80 degrees. Slight temperature fluctuations in the liquids you are using for each loaf could be affecting how much they rise. 
- The freshness of the yeast. Old yeast is less active than fresh yeast. If you are using different packets of yeast for each loaf it could also affect how much the bread rises. You can help your yeast stay fresh longer by storing it in the refrigerator or freezer. 
- Humidity. If the air in your home is humid, it could add extra moisture to the dough. The more moisture dough has, the higher it rises. You may need to slightly decrease the amount of liquid that you use on humid days to account for this difference. 
- Too much or not enough salt. Salt inhibits yeast activity. If you add too much it can keep the loaf from rising as high as it should. If, on the other hand, you don't add enough, the loaf can rise too high. If your salt measurements are even slightly off from one loaf to the next, it could make a difference in how much they rise.
- The temperature of the room. Despite the fact that the dough is protected inside the bread machine, the temperature of the surrounding room can still play a role in how high it rises. Try to keep the temperature in the room around 65 to 75 degrees F. Also, make sure that the bread machine is located away from any drafts. 
The following video has some more suggestions on why bread machine dough may fail to rise as high as expected:
Hopefully some of these suggestions help you get more consistent results. Happy baking!