Boiler not firing

[shortfrank]

Hi all. My first post so please bear with me.
Ran out of oil and refilled but can't fire up boiler. Have cleaned photocell and electrodes and all fine. Noticed that my Tiger Loop had a very low level of oil in it when attempting to fire and between attempts completely emptied. I took off the pipe from the tiger loop to the pump and there is no oil coming out of the tiger loop. There is a non-return valve (I think) just before the tiger loop. I opened the bleed screw and got some air and oil bubbling out but then it just stopped. The burner turns over and then I hear the click but no attempt to fire.
Sorry for the non technical language. I hope you can understand what I am trying to say.
Any help would be gratefully appreciated.

 

[Boilerman2]

Have a look at any filter between the oil tank and the tiger loop, it sounds like a blocked filter to me!!

 

[Kingsway]

Air lock