Could be several things or a combination of all.
1) You may need to raise the Tackling slider for Human on Defense section. But I have mine at 0 on All-Pro and I can tackle just fine.
2) You could have the Run Ability too high. The higher it is the better the runners can do special moves, break tackles, secure the ball, etc. I have mine at 25 Human and 15 for the CPU on All-Pro.
3) The player you're controlling on defense may be a poor tackler.
4) The player on offense you're trying to tackle may have a high Break Tackle, Stiff Arm, or any other attribute that is allowing him to break free so much.
I'm using the sliders from this thread,
Jay's Realism Sliders for All-Pro (TESTED X360). They work pretty good.