DirectDraw-Problems

I''m currently coding on a 2D-RPG based on DirectDraw and I''ve got a strange problem: During the loading process I create several 16Bit-Surfaces...one of them is sized 550x75 and holds the Ground-Textures, so far so good BUT: When I clear the surface (lock it, use memset with 550x75x2 and unlock it) it isn''t fully cleared! I took two screenshots from the bar, both show the bug in the lower part of the image...the light blue thing is my ingame mouse cursor, the dark blue square is one of the ground textures and the lowest square (orange in the middle, yellow in the corners) is a texture from a test I did before and was still in the memory... On the right shot I used memset with a green color to show how far the clearing goes... Does anybody have an idea, why this happens? Thx Chefbrenner