Send them a PM asking if they are there or PM them on GTP to let them know, if they are not responding to PM's, they are likely to be away, if they reply the next day or later on, send to them, if you have other trades booked and need to carry on, just send to next trade and let the person you have agreed with to trade with, trade another time (i.e tomorrow) or whenever they are next available. hope this helps 👍