Please ensure your charging case is itself charged. The five LEDs on the front of the case indicate the remaining battery life.
Make sure that the LED indicators on the earbuds are flashing white whilst in the case. The earbud status light will pulse white when charging and will switch off when fully charged.
If the LEDs are not flashing, you may need to reposition them within the charging case to obtain a connection with the charging pins.
Check that there is no obstruction in the charging case stopping the earbud from becoming properly seated. Check that the earbud tip is fitted correctly. If there is sweat, liquid or obstruction on the charging contacts within the case, corrosion may occur.
In this situation, please clean the charging terminals or earphones with a suitable cloth or alcohol cleaning pad before charging.