Hi Jsbraga,
Many have suggested checking the usual culprits that cause electric bow thrusters to stop working during operation (poor connections, insufficient wire gauge from voltage source to thruster, low source voltage, etc.). These are the obvious culprits. Should you evaluate these (ideally with the help of a marine electrician), and not find any obvious faults, you may find that the thruster motor itself requires service.
The motor is a simple DC motor, irregardless of voltage. It has bearings, armature windings, brushes, a commutator, etc. ALL of these items can and will deteriorate should the motor be overheated, if the brushes are neglected, or the source voltage allowed to degrade, the motor allowed to get wet, and many other factors.
To service the motor itself, it should be removed from the boat and evaluated on the test bench of a competent DC motor shop. It isn't necessary to find a marine electrical repair facility, as it's fundamentally the same motor as a car starter motor. Service for DC motors can be found virtually anywhere.
Evidence of overheat on the motor is usually pretty obvious. The motor shop will megger the armature windings, inspect the commutator, inspect and/or replace the brushes, and also the bearings found on the motor shaft. They should also repair any burned or discolored insulation on the armature, and in general, return the thruster good as new.
As I'm sure you've perceived from the responses to date, many of the issues with bow thrusters are operator-related. The buggers are NOT intended for continuous use, and are equipped with thermal overload switches accordingly. And, if your boat's properly fused, you may find the fuses themselves speaking to you by blowing at inopportune moments.
Unless your Selene is a fairly new model, I'm pretty sure your bow thruster is simply bang-bang controlled. That is, 100% on/100% off. No modulation of voltage and/or current (other than that provided by Ohms law) is typically provided by the thruster controller. No issue, assuming the system is properly designed in the 1st place (probably just fine, as Selene typically does a good job on the initial design and install), and no issue if the bow thruster "system" is properly maintained, and no issue if the bow thruster is properly operated. But there probably aren't any simple fixes either.
Regards,
Pete