I think the blades may have twisted. I'm always doing that to my secateurs because I try to cut too thick branches at the wrong angle using too much force instead of using loppers. I'm afraid it may not be possible to rectify them but you could try taking them apart, oiling them and then re-assembling them. Hope it works.
I think they may be a geared type, so there may be something trapped between the gear teeth hidden under the plastic. Good luck with dismantling them - I know they have a poor reputation as far as the plastic handles go, which are prone to cracking and breaking according to some reviews, so don't force anything.
A trowel in the hand is worth a thousand lost under a bush.
Digressing a bit, but it might be useful information.
I like Fiskars products.
I've a Fiskars tree pruner, the one where you have a length of cord you pull on to close the jaws.
This was OK until some trees got a lot taller. So instead of buying a longer lopper, I bought some dowel of the same diameter as the pole and putting them end to end, passed a fiver foot length of polypipe over both, with them butting up in the middle of the pipe, then securing both ends neatly with duct tape. This keeps it all really rigid.
Adding more cord to the original, the reach is twice what it was.
As a follow up to the above, I contacted Fiskars for advice. They asked me for a photograph of the device and they promptly sent me a brand new pair. How's that for good service. Well done Fiskars!!
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I'm afraid it may not be possible to rectify them but you could try taking them apart, oiling them and then re-assembling them. Hope it works.