My boys have been munching on lab blocks from babyhood but my friend has had much more difficulty with her two boys which I'm currently looking after. To keep her boys interested interested in their food I only offer them a couple of fresh blocks morning & night of 2 different brands (the blocks are kept in air tight containers to keep them fresh) and I keep varying what I give along side the blocks, some times cereal, some times seed, greens of course and if they're very lucky a little bit of honey on wholemeal toast. They munch through the yummy stuff first then start gingerly trying the lab blocks. They seem to have accepted it as part of their meals with persistance and I've spotted partly eaten lab blocks sneakily snashed away which I take as a great sign.
I think persistance and variety are the key because my friend swears her boys refuse to use a litter tray or aren't interested in food enough to train but I've successfully got them litter trained and loving their food now.
Keep food fun and don't give up! I go in to the kitchen and experiment with ways to serve the same old food in a different way and the furries seem to really respond. When my boys had colds I mixed a bit of honey and echinacea together and served on chopped up lab blocks - they loved it.