Build plate cools during print
Your bed is obviously capable of heating up, so I would double check your cable for any kinks, cuts, blow-outs, or general connection issues both where your machine rests during warm-up and Z0 where your machine begins printing.

Most likely there is a poor connector or kinked/cut wire for the build plate.

If that doesn't appear to be the issue, I might also suggest checking your power supply. I've heard of other similar machines' power supplies not being quite strong enough to support two extruders AND a heated bed.