clam trouble

[tinman]

hey i need some advice on my tridanca clam his shell is open but he will not expand and seems really drawn into his shell. Although he does open and close on occasion and he is also not attached to anything the gland appears to be drawn up inside the shell he has been in the tank for roughly 3 months and doing fine till last weekend. I have read that some soft coral can irritate the clam or sting the clam I do not have any stinging type coral in the tank. my water quality passes the the test. and the light is real good my other clam seems to be expanding well and is showing not signs of stress any ideas would be welcome?