Have you checked yoir water stats lately fishfan? you mention that one of them has died and I think corys are sensitive to water chemistry so it might be worth checking that. Make sure the filter is working well too.
As for behaviour, as far as I am aware corys are quite active and gregarious, although do have a habit of playing dead just to make sure you are paying attention
TROPICAL TANK-Elite Tank - 207litres
6 Harlequin Rasboras, 5 Emerald Eye Rasboras, 10 Cherry Barbs, 2 Bristlenose Plecs, 2 Panda Corys. 4 Dwarf Chain Loaches.
ViaAqua Tank AR620 97litres
1 red male betta, 1 BN