I have a cargo wagon setup with roboport, expanders, batteries, etc in grid and materials and construction robots in inventory. When I send the train to destination and lay down a blueprint, it doesn't reliably deploy the bots. Every once in awhile, if I move the train, it will deploy a few bots, but once they return, it doesn't send them out again. Any ideas?

This is an issue that I've noticed myself, and my only real conclusion is that it's an issue with the game engine itself.
The mod just uses all stock game engine functions to let you do this, so it's a quirk on how Roboports and blueprints work.

If you notice when you have a big roboport grid, and set up a mega-factory sized blueprint, without any robots in the network, it starts to complain that there's not enough robots, but tops out at about 600, even though there's many times more jobs to do. This is because the jobs are all poled in a specific order, and if there's no roboport in range, just don't report an issue.

So, you place your blueprint down first, then move the vehicle into position... you could have missed the pole to request a robot, as your train wasn't in range, so it could take a while before the request comes around again, especially if there's a lot of ghosts around your map.

A better way to do it is to get your train into position first, then place the blueprint. Every available robot on your train will go to a single job, but then you'll run into a similar issue. So, you'll need to use smaller blueprints, and go at a steady pace.

I am kind of curious as to if the Spidertron has a similar issue, as it exists in vanilla, but I have noticed similar issues even just using the player's suit.