Very often, when I try to add a fuel tank or engine below an existing component, it attaches sideways. What am I doing wrong?

I recently downloaded @retrorockets' Mercury space capsule add-on, because I thought it'd be interesting to combinre it with various configurations of fuel tanks and engines to see what happens. But whenever I try to put a fuel tank below the capsule, I get three tanks sticking out horizontally from the bottom of the Mercury spacecraft. In this case I think it may be related to the fact there are three retrorockets on the capsule, but I have a similar issue with attaching engines with just the default components. Usually, when it happens, the engine sticks out sideways near the bottom of the tank, like a gigantic RCS engine, but there's only one rather than three. I still can't complete the build tutorial because this always happens.

How do I prevent this?