My mountain bicycle has 3 chainrings at the front and 7 cogs at the back, making it a 21 gears bike, which are shifted by a derailleur system.
Although it is 25 years old, everything seems to work fine, except for switching gears down from 3-7 to 3-6 and from 3-6 to 3-5. (With X-Y I mean Xth ring, Yth cog.) Whenever I try one of these two particular shifts, they take longer to kick in and in the meantime I hears "clicks" from the mechanism, differently from the other shifts, which take effect fast and smoothly.
The same behavior also happens in another 21 gears bicycle I sometimes ride, which leads me to the question:
Is this effect widespread and normal or is it a malfunction I'd better repair?