Hey Dazza, great to see Josh's (Rpul's) piraiba with you and doing well. The feeding habits you describe sound about like what I see here. It changes with fish age, size, tank mates and comfort level.
The hesitation appears to always be connected with less comfort. I have to stand on my knees and beg my 4' Suriname piraiba to feed sometimes. He will circle and swim around for 10-20 min before deciding to partake. This is because he is bothered by the 2.5'-3' Sperata aor all the time and is in general skittish and takes a back set to most other catfish tankmates.
Two other piraiba (Peru) are in the other 4500 gal and one is dominant and feeds well while the other is submissive and feeds poor.