Good evening !
I would say that your jet is probably too much in, and does not allow enough air to go in. I may be wrong, but I would suggest that you pull it out a bit and check.
The best way to see if your burner is functioning good, is to pull it out of the boiler and to check if the flare is fairly strong and blue (no yellow in the flare exept (eventually) at the tip). Also, if the burning gas out of the chemney is picking your eyes , it means that you have not enough air.
I shall try to get a photo of my mcSteam boiler tomorrow to compare with yours.
The right solution (as Kiwimodellers says) is also to contact Micke from McSteam who is always of great helps.
Last, I won't keep the gas tank insulated. It needs heat exchange to keep some pressure inside. Your problem may come from there also. You may have enough pressure at the begining to supply gas to the burner, but the tank cooling down, the pressure drops, and there is only a weak blow to the burner.
Last: a McSteam boiler works good and your will do as well.