You could be up against some sort of turtles, but even if you are not, this is not at all uncommon. The median time for a wave is about 18 minutes at difficulty 7, but it can vary to something like 2.5 of that, or that divided by 2.5. At any rate, if can easily happen that on diff 7 you might not have a wave until 40 minutes in, and on lower difficulties it could be the same. But, when this happens, those waves are equivalently stronger. So if the waves are further apart, they are bigger, and if they are closer together, they are weaker. Even if the first waves in a game are slower, that doesn't mean they won't then be followed up by another much smaller wave (say) 8 minutes later. In general, there's a lot of variance in the frequency of when waves will arrive.
