I'm trying to print with PLA on a new Wanhao Duplicator i3 Mini. In regular temperature settings (200-210°C) the printer underextrudes and the extruder motor skips, which led me to suspect low nozzle temperature. Increasing the temperature to 225°C indeed allows the printer to print normally without any problems.
What can be the cause for this? Is it a faulty temperature reading? What is the way to fix it?
It's very difficult to diagnose 3D printers without physical access to the machine, but here's a few possibilities that come to mind (several of those may contribute together to the problem).
The temperature reading is poorly calibrated or defective
This means that the actual temperature is lower than what displayed. You can verify this hypothesis by using an IR thermometer or replacing the probe (on some printers the heat probe on the bed is the same than the one on the hot end, so you don't need to buy a new part).
The batch/brand of filament you have has a higher melting point than expected
So: the filament is supposed needs to be extruded at 225°C, but the label (or simply your assumption from having used other spools of filament) says otherwise. Just use that temperature and be happy.
The teeth of the extruder gear are worn
Within limits, the hotter the molten plastic is, the runnier. This means that at 225°C it will be less viscous than at 210°C and will strain the extruder motor less. If the gear of your extruder has worn teeth that won't "bite" deep enough into the filament, they will eventually slip when reaching a certain torque. The teeth of an extruder wheel should feel well defined under your fingers, if you are in doubt about them not being sharp enough, then they probably aren't, as the feeling is quite distinctive. Replacement gears can be purchased online very cheaply. EDIT: re-reading your question I realised you wrote that the motor skips, so this and the following point may not be the case for you, even if sometimes - but not always - slippage makes a similar noise to skipping.
The spring of the extruder gear is weak
See above, but here the root cause of the slippage is the extruder gear not being pushed hard enough into the filament. This is caused by the extruder spring being too weak. Most printers have some mechanism to regulate the tension of the spring. A DIY method is that of adding a stack of washers (or a custom printed cylinder) at one end of the spring in order to "pre-tension" it. Otherwise again: spring replacements are extremely cheap.
Your part fan is blowing on the nozzle
The temperature probe measures temperature at some place in the heat block, not the nozzle. If your part fan is blowing too high, the flow of air may be cooling down the nozzle, instead of the extruder part. You most definitively want to avoid this by repositioning the fan or printing a custom fan deflector (the latter is a classic among mods). This is an easy-to-diagnose problem as most slicers allow you to turn the part fan off.