Have you noticed what the fish were taking off the surface? Might find a couple of clues there.
I think you might have a problem with timing. Try to hit the spot just as it gets light or just as it gets dark. Use plugs then.
I've noticed that when targeting Mahseer that shoal only the first couple of casts produce fish before the rest of the shoal spooks. If you do land a fish, wait 10 mins before you cast out again.... let things return to normal. Large shoals of Mahseer can be quite frustrating/picky.
If plugs and lures fail, try live bait (small carnatic carp or similar) or even crab. They should provide you with result as they're natural food for Mahseer.
Using bait would need you to ground bait before hand which may be time consuming for you, but it's an option.
Let us know how you get along and post some pictures if you do catch some Mahseer.