Copper plating not bonding to steel object

Here's four possibilities:
First, the knife may actually be stainless steel in which case it is impossible for you to plate it in this casual fashion.
Second, the knife isn't clean. Wear rubber gloves [on eBay or Amazon] and scrub it with a brush and pumice [on eBay or Amazon] and detergent or Soft Scrub [affil links].
Third, the knife is steel and it is clean, and this is the best you are going to do from a copper sulphate bath because copper "immersion deposits" on steel.
Fourth, if the knife is steel and it is clean you should be able to somewhat improve the adhesion and limit the non-adherent immersion deposit via "hot entry". That means attach the wire after cleaning, and turn the power on, and make sure the steel does not see the copper sulphate solution except when it is already electrified.

I'm not sure of the question. Are you plating knife blades with copper or do you want to prevent rust? Delete the sulfuric acid from your plating solution. Immerse the steel in a phosphoric solution (available from Hardware or Home Centers as an etchant/metal-prep) for about 15 minutes. Rinse well then plate. DO NOT touch the surface before plating.

If you want to keep the blade from rusting, use polished stainless steel. You can also polish hot-rolled and cold-rolled steel that will maintain a nice look for a long time. Longer, if you can keep a light coat of oil on it. I've polished hot-rolled steel to a chrome-look finish for tool parts. These have lasted for years with a light coat of oil.