All mine seem to remain on 'High' even when not being used for days at a time. I do have the Power Save turned ON.
the solar ones seem to really like the direct sunlight on mine they charge up fast in direct sunlight. if you can find some sunlight for a couple of hours it should charge right up. keep the auto el off though it eats the battery up on mine. if it doesnt charge ask casioi guess.... but it should work. did u just recently purchase this watch?