When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. (PHB, Ch9, Actions in Combat)
Does that mean that by ignoring the trigger you don't take reaction, and therefore can take it the next time the trigger occurs? Or is your readied action wasted?
E.g. a mage is ready to blast any enemy charging through the door, but it's just a lowly xvart. Can she skip to the next one?