Te buttons are there fro water conservation. The smaller button the shorter the flush and therefore saving water. This especially helps if you are on a water meter. You can continue using the big button but you will be wasting water...depends on how green you are :)
Yea sounds like the little plastic tube underneath has come adrift
Remove button carefully and reattach ,it's ok to use the big
Button of course that gives you a 7 ltr flush the small one
Is for short flush 3 ltrs
I would firstly check the contact on the rear of the button to make sure it reaches the flush mechanism in the systern.
If not remove and replace very simple.
If it does reach then it will be the flush mechanism in the systern that needs replacing not as easy but still fairly simple.
These can be a pest some work on a capillary wire and most common are 2 lugs which depress under the button it's an easy mistake to cut these lugs short have a look at that first
Remove cistern lid and depress manually