Volvo D12 occasionally won't start on first second and third attempt.

Hey guys I have a minor issue with my truck. It runs great, I have no fault codes, I have good power and good fuel mileage per gallon. On occasion when starting the engine it will just crank and crank and not fire. Usually when I try again it will fire right up like nothing ever happened and sometimes it takes a third attempt. I do always put the key into the on position for about 5 seconds before I turn it further to engage the starter. Someone recommended to replace the crank sensor, I had that replaced although still having this issue on occasion. Many times it will start with no issue but there are moments where it takes up to 3 attempts. Is this anything to worry about?

 

Thank you for leading me in the right direction!

 

The problem actually fixed itself. I didn't do anything and after a week it happened less frequently and ever since then it starts on demand every time.