I also have Corona and Fiskars loppers and have the same problem. Finally, I gave up and bought some Felcos. They seem to be better but I haven't used them long enough to tell if they will hold up. They do seem to be really well built and the replaceable blades should be a plus.

You probably cutting too big limbs especially if they are not double hinging type. Only thought would be check the bolt and nut holding them togather maybe it has worked loose? Seem sharp? I would also sharpen and watch closely when the close looking for gaps from your poor work habits.

Twisting in the cut is a bad thing. There are times I'll take a cut at an angle then remove the tool and bite from another angle to remove a chunk out as if using an axe. This doesn't put side stress on the blade. Other than checking for the loose bolt the tool may be scrap. Mine are the bypass type where the blade does not go against an anvil. Most anvils can be adjusted so the blade and anvil are in contact all along the cutting length when closed.

Are you talking about actual 'Loppers', with the long handles, or are you talking about trimming shears,which
are held by one hand? There are two types of each,
the anvil type, (which it sounds as if you have), and
the 'bypass' type, which is better for green wood.
Google them both and you'll see what I mean.

Hold the loppers up to the light: Can you see any gap between the blades.

Operate them. Do they make contact through the entire stroke?

Things to try:

1. Tighten the nut Combine this with a dab of grease between the blades where they overlap.
   
2. Sharpen the blade, but only sharpen the outside edge. If you sharpen the inside edge, the edge won't meet the bypass. This should be finished with a very find stone.
   
3. Bend the blade. Not much. Bend at the base by putting the whole blade in a vise then using a hammer on it. You want ot make a tiny slope so that the blades are forced together.