First of all, I would check for any traces of dirt on the wheels and track. If everything is clean, it may be worth dismantling the loco. Looking at the exploded parts diagram for the 08, there appears to be leaf springs pressing down on the top of the wheels to make the contact. You could check if these springs are clean and have a reasonable downward tension on the wheels. If necessary, you could try adjusting the spring fingers by carefully bending down slightly. If you decide to dismantle, take care with the body release clip at the rear underneath as this is easy to break.
Hope this helps.