Solenoid.  The box is wrong.  Glad you found out the voltage drop.  I'm surprised they'd work at all at 11 volts.  Mine need a little help to open (poke a bolt up through the hole in the bottom and push them open) when the voltage drops close to 12.  Not sure what your wiring setup looks like but sometimes I find it faster to just replace all of the wires, connections, etc rather than try to find the bad spot, if the trouble isn't in obvious places like the connectors or the fuse holder.  14ga works well for me.  16ga not so much.