I bought a small Venus flytrap from one of the RHS shops, and was told that it could catch bluebottles. After bringing it home I reported it in a nice large pot with specialised carnivorous plant compost, and kept it moist constantly. A few weeks later, it's grown 2-3 times larger and appears healthy, and has grown a huge flower spike too. But I've never seen it catch anything, big or small. This morning I killed a fly and actually put it on an open leaf, and nothing happened! What's going on?