You definitely have a leak.
Your bottle should dispense far more than 2 kegs alone. Given perfect circumstances, it normally takes around 1 lb of CO2 to dispense a 15.5 gal keg. So, if you normally use 5 gal cornies, then you should be able to dispense at least 15 kegs from 1 full 5 lb bottle of CO2.
Now this is considering that everything is ideal in your setup. (your system is airtight with no leakage, minimal headspace when carbing kegs, etc.) I have yet to get this number of kegs out of one 5 gal bottle, but 10 kegs should be easily attainable.