How do you get your tanks to engage when they wont

Position yourself to follow them up.

God knows I'm probably not going to engage if the rest of my team is too far behind me to follow up before I get nuked down.

Promise you'll follow up is a good start.

Poke the enemy down a bit first. And don't stand so far back. Weave back and forth with their movement. Gives em a feeling you will follow. Becaise ya know, you are.

Play tahm kench, eat them, flash in to the fray.

Please don't actually do this.

usually because you dont follow up so you lost their trust