It usually happens when the battery is completely empty. When the battery is completely empty, it would not be correct to charge it in the ordinary way. For this reason the instrument protects the battery by charging it very slowly. In this case:
1) leave the instrument on charge for 6 hrs about;
2) disconnect the charging cable and wait 10 minutes;
3) put on charge again: if the instrument’s display flashes, complete a full charge (12 hrs about).