The fact is - all long-term cast members have contracts that stipulate a number of appearences in the length of their contract - usuallly twelve months. This means that story-lines revolve around different characters at all times. If you analyse it, you'll find that all characters are absent for varying amounts of time as stories evolve - it's the way all soap contracts work. It has the double bonus of variable stories, and allowing actors to go off and do other things if they wish.