Assuming you purchased the digitizer with the IC already soldered onto it you could be looking at several different problems. Check these trouble spots first.
First, check the digitizer IC socket on the motherboard, examine the pins inside the socket itself. If any of those pins look : dirty, not sitting even with the others, missing or damaged in any way this will render the touch input completely useless. A dirty pin can be cleaned using isopropyl alcohol and a q tip, anything other than a dirty pin will more than likely require that socket to be replaced by a professional.
Second is to make sure you transferred the the tape on the home button ribbon from the old screen to the new screen. I havent seen this issue affect the touch input but it will occasionally mess with the lock and home button operations.
Third, when sealing the new screen to the frame be very specific about how you feed the cable down into the frame, aftermarket digitizer ribbons are fickle when it comes to bends or creases. Some will work after being creased some will flat out refuse to work or cause "ghost input". Basically none of the cable should be pinched between the frame and glass once you lay the glass flat on the frame and it should be fed in a "S" style while doing so. If it is already creased its a fair bet youll need a new one.
I hope this helps and good luck getting your iPad mini working!