chris covered it pretty well, but i thought i would just add a few things

i play pretty much the same stakes tourneys as you, and my ITM hovers around the 19-21% mark. having said that though, i have had significant winning months where my ROI is 10% and losing months when it is 25%

i tend to play smaller fields given i play in australian night times and i also play quite a few small field tourneys on tilt and 888 so i would think my ITM would be higher than people who play exclusively big field stars tourneys, but in general i dont think its a stat you should pay too much attention to – it matters MUCH more what you do in the mid to late stages than it does on your cash percentage. you could bubble 20 tourneys but then win one and that would make much more of a difference than 20 min cashes 

so i would advise not to worry about your ITM, being around 20% is great, but as long as its not 5% then you are at least making it semi deep into tournaments, but you might have some other leaks, either in your push-fold game or maybe you are too tight on the bubble or something else.