Arduino Uno with 6x 5V solenoids + 9V battery - Not enough power?
I'm using a 9V battery connected to the DC input


Well, there's your problem. Those 9V PP3 batteries are barely enough to power the Arduino by itself. Add anything like motors and solenoids and you're going to have problems. Big problems.

Ditch the PP3 battery. It's only suitable for very low current devices, like smoke alarms. For anything more you need at least multiple series AA batteries.

Also the solenoids must be powered direct from your power source (or through their own regulator if needed), since the Arduino's on-board regulator will go into thermal shutdown quite quickly with 9V in and a large current draw.