Walk Behind Lawn Mowers

I purchased a 775H last year new. I am very satisfied with everything except the blade. I have had many mowers over the years and have never had a cutting problem until now. When it was new last year I had to re-cut many areas and also followed the owners manual and had it professionally sharpened this year before mowing and had to re-cut many areas again. Can you please help me or tell me what I may have done wrong?