Rejected blocks when only one miner on the network

[petronius]

I cloned bitcoin to mess around with the code a bit and test out some mining configurations on a fake coin with a low difficulty. Everything is working smoothly but when I mine with cgminer (3.7.2) I am getting a lot of rejected blocks (about 30%). There is only one machine mining on the network. What would cause the blocks to be rejected if there is no competition on the network? Is this normal?