Hi Madila
These nickel/graphite coatings are sensitive to changes in spraying parameters and the slight variation in powder properties you get from different batches of powder (thorough tumbling of powder before use is also important). This reflects significantly on the hardness and abradability properties of the coatings. It is generally accepted that regular hardness testing should be done to first establish the optimum spraying parameters to achieve the desired hardness level. Usually done by adjusting the powder feed rate only (Metco 6P gun, higher gives softer, lower gives harder) or adjusting oxygen and fuel flows simultaneously only (Metco 5P guns, lower gives softer, higher gives harder). Hardness testing and "tweaking" parameters does need to be done on a regular basis and particularly when any changes are made like powder batch or equipment settings.
Another area that may be worth looking at is the hardness testing itself. Make sure the recommended procedures are carried out correctly. Coating thickness and preparing the coating for testing are important factors.
Hope this helps, but suspect you probably already know this anyway.