Following up is hard -- you don't want to make the other person feel bad/make them feel like they made a mistake or come out sounding too aggressive
I would love a template on how to follow up politely
e.g.
I love this question! The core principle to remember here is to put yourself in their shoes (i.e. have empathy) and then weave that into the words you say.
For example, let's say my tech lead said he would build some API for me, but he's now 2 days late. Tech leads are really busy, so I would say something like this:
Hey [Tech Lead], sorry for the extra ping (I know that you're super busy), but I wanted to follow up on the status of API X? If we don't get it by Monday, we'll probably miss the next milestone. If you can't make it, just let me know and I'll talk with our manager to figure out a backup plan.
In this message, you can see that I'm being very polite and appreciative of their situation, but I'm also making it clear the damage they're doing to me and the team by falling behind (just stating facts, not rubbing it in their face). There's also a lot of tentative language, and I'm trying to remove as much of the burden from them as possible (you can see that I take ownership of covering for them if they can't make it).
All of these tactics (and far more) are covered in our Effective Communication course: [Course] Effective Communication For Engineers
Here are some tactics: