Peroxide determination of canned tuna' s oil

Hi everybody! I had some problems today in lab. Yesterday I did the determination of peroxide value of olive oil, according to the official procedure. It was fine.
Today, I tried the same procedure on olive oil from canned tuna ( I just took the oil from the jar). Bad results. The sample didn’t become blue.
The reagents were fresh.
I thought that something in the tuna changed or reacted in a mysterious way, because the procedure is fine without tuna.
Anybody has experience with this type of analysis? Thanks you!

In theory ,Its inevitable that unsaturated jfish fatty acids are susceptible to lipid oxidation due to heat and catalytic metals contact as well as inherent heavy metals in fish flesh and during cooking…
Concomitantly those oxidized fish fatty acids will enhance the peroxide value of clean vegetable oil used as a vehicle for fish like tuna.