DS is 18 months, and just recently started signing. We only use the ASL "toilet" sign, and we know he has to poop because he's WAY more insistant upon it! "Mama!" Signs potty. Pauses. MAMA! Signs again. Lol. Also, he'll sometimes get my attention and do a little grunt, so that I know he has to poop! So, i wouldn't worry about it. We only taught the ASL sign half heartedly, and he still has caught on and can tell us when he needs to go.
And I don't think it would matter if you changed the sign you were using, we tried the tapping the chest sign for months and he didn't catch on, but as soon as I made the potty sign he was so excited and did it right away!
Mama of two... DS born at 35w5d (11/06) and DD : born full term 38w3d (5/09) on what would have been my dad's 64th birthday. Always missing my dad who died of oral cancer 3/11/09.