I have a Ventus B.

You have plenty of suggestions about the mechanism back at the wheel well.

However, the mechanism in the cockpit behind the seat pan can get dirty and create the same kind of problem.

There is a tube in a tube sliding assembly plus several support bearings. Once clean use a lubricant like the LPS family. LPS-1 is a dry residue lubricate. LPS-2 is a wet residue lubricant. Either can be used as the cleaner with a shop rag.

For the final lube use the dry lube if you are in a dry, dusty area. Consider the wet lube only if you are in a humid area. In either case a little lube goes a long way.