I tried placing them in Twikkii Island and they wouldn't place there, either. But one small beach lot I got from MTS2 did place there fine, but wouldn't place on my Sleepycat beach hood at all. So I'm thinking that beach lots are very touchy, and place or don't place depending on their size, the beach shape, and maybe the phase of the moon.
ETA: I did some checking around, and it does appear that sharing beach front lots will be a pain in the butt. The terrain the lot is placed on has to very closely match the terrain it was made on (i.e., slope), or it just won't work. Which explains why I could get one lot to place in one hood, a different one in a different hood, and neither of yours anywhere.
Oh well.