Gert,
The first thing to do of course is to pull out the ejectors themselves and make sure that the stems aren't bent or dirty. The blades could also bind up if there is any dirt present. Either of these conditions will slow down the kick of the ejectors. A very little bit of dirt between the two halves of the ejector stems will cause them to be too tight in the hole they fit into. Anything that is bent even a little bit will need to be carefully straightened.
As Eagle says above, you also need to inspect for weak ejector springs. Someone who really knows ejectors could probably remove the springs, if they are flat springs, anneal them to a soft state, and re-harden and temper them back to an "as new" state of power.
I am always amazed when I find an otherwise clean and oiled gun where the owner has never thought to remove the ejectors or the extractors, as the case may be, and clean out the hole or the stems to make them work better. Another place that is regularly missed in cleaning is the bottom of the blind hole in some guns where the Greener crossbolt operates.
If you feel up to doing the job, I would suggest that you carefully dismantle the newly accuired Greener and thoroughly clean and lightly oil all the parts, then put her back together again and see if there was any improvement. If you are not familiar with ejector mechanisms, I would suggest that you let a trained gunsmith work on the forend part of the mechanism. Ejectors and their timing, are kind of a speciality area, best left to the trained experts, I think. Good luck with your project. Bob