Horrible title - but here's the gist of it.

In the campaign (not sure if it matters) - I created a unit that cost some gold and 25 materials

I was able to train one of them - but didn't have 50 materials, so the option to train 'grayed out'.

I left the train selection open

Hit next turn a few times.  I noticed I had more than 25 materials again - but the button was still 'grayed out'.

I tried selecting another unit and then back, and that didn't fix it.

I was able to train the unit after I closed/re-opened the city management screen.

Sorry, no screen-shot - as I don't have screen-shot tools.

I was able to reproduce this.