Bro, dun worry that your sun dun open immediately when u bought them. Perhaps u should try cyclofreez mixed with the phyto & zoosplanktons.
Some sunnies take a while to acclimatize. They may be stressed due to transportation. For new sunnies, they are not as sticky when I feed mysis or brine shrimps. After a week or two, they will be very sticky to catch any passing food. Of course, your water parameters must be within healthy range.