How to re-magnetise an old magnet ?
Re-magnetise Old Magnet
Even if your magnet is kept in top condition, it can still become demagnetised during its lifetime. An example of this is a samarium cobalt magnet which has been shown to naturally lose 1% of its magnetic capabilities over a period of 10 years.

blue-and-red-rectangle-bar-magnet
To overcome losses of magnetic power, all magnets can be re-magnetised with the help of a strong neodymium magnet, which has the strongest magnetic field.
To do this you will need to follow these steps
Step 1 – Rub magnet north to south

rub-magnet-north-to-south
If the magnet has a keeper, you should remove it before proceeding with the re-magnetisation.
Next, rub the magnet from the north pole to the south pole with the north pole of the neodymium magnet several times.
Step 2 – Rub magnet south to north

rub-magnet-south-to-north
Next, rub the magnet from the south pole to the north pole with the south pole of the neodymium magnet several times.
Your old magnet should now be fully functional again.