Weak starter

I had to refrigerate my starter when I went out on business for a week. I came back and have been feeding it but it seems weak. I have tried to do a preferment and it either does not work (1/2 cup AP flour/1/2 H20, i Tbl starter).  I have waited for 12 hours and the preferement is still weak or not working. Fails the float test. How do I strengthen the starter to get it working bette?The starter is fed once day with 100 gms AP + white wheat at 50/50 and 120 g H2).