In making award flight reservations recently, I encountered trouble (again)in reserving the flights I really wanted. I had no problems in getting my departing (westbound) flights, but had to take early morning flights on the return (eastbound). This case happens to be on United but I have also experienced the same thing on American. My departing flights usually have been on Friday evening and my returns on Sunday morning. The flights have been reserved well in advance (6-9 months) and in some cases award seats have not even been allocated yet on the desired flights. In some cases no award seats were allocated even one month prior to departure. The destinations are continental US and to less than exotic destinations (Denver, Rapid City, Las Vegas to name a few). Has anyone else experienced more difficulty in claiming free seats on eastbound US flights? Any ideas on why this might be so?