You say you have clay soil deep down. How far down? Is it affecting your drainage? You say you amend the soil. Did you just amend the planting hole? If so, and if it is surrounded by clay, then it will become a sinkhole and trap extra moisture. Your best bet is to amend the entire bed, not just the planting hole.
Also, have you done a soil test? Azaleas like an acid soil, below 7 pH, more like 6 or 6.5 pH. You can easily test with a kit from the garden center. If it's 7 or above, acidify the soil with Miracid, Acid-Gro, or similar product. You can also mulch with coffee grounds and pine straw.
With fertilizer, make sure you're getting a balanced feed with N-P-K. Heavy nitrogen actually works against flowering, causing more leaf and stem growth instead. Make sure your soil has Phosphorous available for flowering.
Luke Miller, editor
Garden Ideas & Outdoor Living magazine