Your texture seems to be using masking, instead of alpha. When you create the texture, make sure you use the alpha flag.
Texture masking works on the basis of the number being above 128 being poaque and less then being transparent, as you have found out. But, probably because you are using the mask flag, instead of the alpha flag.
Texture masking works on the basis of the number being above 128 being poaque and less then being transparent, as you have found out. But, probably because you are using the mask flag, instead of the alpha flag.