Clams that won't attach

[Aquattro]

Tony, how big are these clams? I guess at $50, they're smaller? I know larger clams sometimes do not attach since their weight holds them in place.
Every clam I ever got attached within 2 days, unless they didn't like the location in which case they moved themselves. Genrally I try and place them in a depression in your average liverock and they do fine. Current may be a factor; I understand croceas like more current than maximas. Is it possible something in the tank is pestering them? Large worm maybe?
What type and how close to the light are they? Although I don't think lighting would be a factor in attachment.
Basically I find a calmer area up high with a depression and they go about their business!
I have a crocea coming this week, so I'll try and "time" the attachment