Probably because ABC is VHF in your area and that is what your antenna is designed to receive. Most digital stations will be in the UHF band and need an antenna designed for this. http://www.solidsignal.com sells quality products. I recommend the following antenna if you are located very far from the stations, they make smaller versions too depending on gain needed:
http://www.solidsignal.com/prod_display.asp?PROD=AD-DB8
I also recommend a good preamplifier. I currently am using this one and you can find many other people on video forums who use this as well:
http://www.solidsignal.com/prod_display.asp?PROD=ANC7777
Last but not least, if you have many splitters or very long lengths of especially RG-59 wire, you may need a distribution amplifier. However, I wouldn't get that until you see if these two items helps.
Also, if you haven't already, go to http://www.antennaweb.org and see how far and what direction the stations are in. If they are located all around you you may need a rotater