What's the secret?

Flannel is not described by thread count. It's described by weight, as 4 oz or 5 or 6 oz. If it is not described by weight, it's either not real flannel or it's very cheap flannel. The heavier it is, the better it is, but 6 oz is as heavy as bedsheets will be. Try ordering from one of the online places such as LL Bean or Land's End, which do describe their flannel in terms of weight. Also look for flannel that's made in Germany or Portugal. Any company selling the good stuff is going to tell you where it's from and what weight it is, because those are selling points to justify a higher price. If it's cheaper stuff they'd rather you didn't know. Pilling, which I think is what you mean when you say the sheets get bumps, when you get those annoying little balls of threads stuck to fabric, is far less likely to happen if your sheets are 100% cotton, which all good quality flannel is.