It's in the oven but it's not thick

If you measured the flour by the cup measure and not by the weight measure, you could have added slightly less flour than is required. Some folks naturally scoop slightly less, some slightly more - which is why a weight measurement is more accurate and dependable. If you used extra large eggs instead of large, that can also bring slightly more liquid to the party. What counts most is how the cake tastes, than how the batter looks going in. How did it turn out?

Hmmm. Maybe there is something you measured wrong like more milk than required?