Resolder it. Looks like it needs just a bit more solder too. The kind of solder can also make a difference. I use Kester for mine. Rosin core. I think it's lead free too.
If your terrible at soldering or dont want to do it, any LHS or any local racer/s in the pits should be willing to do it for...
I use a Weller 40watt station and does me well. Could be the solder.
I thing we usually use rosin core solder. I use Kester for the brand. Most solder that come with stations or irons aren't the best.
Hmm.
I would re calibrate the esc.
I would also go into the esc settings, see what LVC is set for. If it set on auto, I would turn it to 3.2 - 3.4v/cell. Auto LVC could be messing it up.
Looks like you still have the clear plastic wrapping around the motor, I would take that off. Just for...