Are you using any mods or cracks? Where do you have the game from? Are you playing with the original DVD in your Drive? Make sure to check out the whole map and see if there aren't any mission icons there. Try going to the place where the High Stakes, Low-rider mission is supposed to start, and maybe you'll find there a red marker, even if the mission doesn't appear on the radar and map. Check out the Mission Tree
to see if there aren't any other missions to complete. Have you tried removing the gta_sa.set file located in the San Andreas User Files folder?
Also, please upload your save file here
so we could take a look at it.