Quick tip since we're on the subject.

For those still using the glass bottles, the easy way to get the ketchup flowing is to tilt the bottle with the opening where you want the ketchup to go. Then bang lightly on the side of the bottle, not the back. I use the knuckle on my middle finger, but feel free to do whatever works for you.

Ketchup isn't quite a non-Newtonian fluid, but it's close. The vibration gets it moving under the air bubble, allowing the air to flow in and breaking the air lock. Hitting the back only pushes against the air lock, making it harder.

Not many glass bottles around these days though. Don't miss them.