To make failure training work, you have to fatigue every possible muscle fiber. Training to failure doesn't mean "go until you're tired." It means "go until you can no longer contract the muscle."
One way to do this is by using old-school drop sets. Start with a weight you can lift 8-12 times, then move to a lighter weight and perform more reps. You can do 2-3 "drops" per set. Keep the rest time between drops to a minimal, basically just long enough to grab a lighter weight and take a deep breath or two.
How Much Weight Do You Drop?
A good rule of thumb is 20% of your starting weight per drop. Use one high-intensity drop set on the last exercise you do for a body part. Any more would be too much.