A little more information would be required before a definite answer can be given.
If the leather is a bicast leather then this is now a fairly common problem on furniture that has not been correctly protected and cleaned. Body oils and dirt can break down the finish on the bicast which will make it go sticky. There are things that can be done (not with floor polish!!) if the breakdown is not too severe.
If you have a pigment coated leather a sticky finish cna be down to the breakdown of the finish on the leather. Once again body oils and dirt are major players but it is often caused by the inappropriate use of cleaning products (baby wipes, washing up liquid, conditioners, furniture polish etc.) which can ll cause the breakdown of the finish. This would need to be rectified and refinished.
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