I often fish stillwater by float fishing.I fish just off bottom using worm,maggot or chopped ham.I catch carp (COMMON,CRUCIAN and tench). But the biggest i have caught was 4 pound so i have trouble catching big fish, is it because i am using 8 pound line straight through.
Actually, I am surprised you are catching small fish with 8lb straight through! Unless good double figure carp are not uncommon you should rarely need more than a 4lb bottom max. If fish generally run to 4lb, then a 4lb mainline and 2.5 - 3lb hooklength should be fine if the water is not snaggy.
Winter time you normally have to scale down your tackle anyway. On stillwaters the fishing becomes much slower due to the fishes metabalism slowing down. They don't have to swim like river fish hence they just slow down, conserve energy and eat less, particularly carp and tench.
On commercial fisheries (sounds like you fish one?) they may be more active due population density and therefore a bit easier to catch. Just carry on with your float method, but fish just on the bottom so the bait is static. But I would consider scaling down to a 3lb bottom, maybe less if big fish are not in evidence, and just feed less. Maggots will continue to catch as will worms. Try chopped worms as a feed to attract fish.