The problem you describe has been a pet peeve of mine since I bought my first combine. A new sickle will always cut well but after a few acres much of the cutting is done by the hold-downs instead of the guards. I have long believed new sickles are bent from the beginning. Either they are bent before installation at the factory or some clod picks up the head with a forklift and bends sickle and skid plates. Cutting problems dont show up until the sickle wears in a little. I have thought of buying a replacement sickle that comes in sections that you bolt together. At the very least, you can throw away any sections that are bent before going through the time and effort of installing them on a head. Every other section upside down sounds promising but I am plain running out of patience trying to re-engineer the sickle industrys problems. Tom in MN