There is a safety feature in the gas tank that stops or restricts amount of gas allowed in case the gas line becomes disengaged or broken. Take the tank off the grill turn it upside down and that valve will reseat. Reattach the tank to the grill and open the tank valve slowly and the grill should work fine. You may not need to have the valve fully open to have sufficient gas pressure for grill to operate. Full tanks have more pressure then 1/2 full tanks. Only open valve half way slowly and check the flames and if not enough open a little more.

You need to remove your valves. Take them apart, clean them with a solvent, lube them with caliper or heat resistant lubricant then reassemble and reinstall.

I had the same problem. Went through all of the recommended steps from Weber to get the excess flow device in the tank to reset. Did not work. Changed to a new tank. Did not work. I concluded that the problem was the pressure regulator on the grille line to burners. Replaced the regulator and line to the burners. Voila! The grille works normally.

Some suggestions for analyzing your situation:
1. You tried 2 tanks and neither fixed the problem. Therefore the problem is between the tank and the burners. The only possibility is the pressure regulator. Note the LPG pressure in the tank is a function of the liquid LPG temperature and does not have anything to do with the liquid level until all of the liquid has vaporized and the tank is emptied.
2. If the problem was with the safety device which limits excess flow when the LPG tank is opened after you opened the burner valves, you should still get a small flow of LPG. This should be enough to get a small flames when you use the igniters. If you can light a flame trying one burner at a time, and you never get any whiff of LPG smell, you are getting zero flow. The pressure regulator is completely blocked. Note since you tried different tanks, you should not have the same excess flow problem unless you opened the burner controls before opening the LPG tank control valve. Even then, one of the tanks should have reset when you followed the correct procedure.
3. Checking the burners for rust blockage is good but is not likely to happen in ALL of your burners at the same time.
4. Taking your burner regulators apart, cleaning in solvent, and lubricating with special lubricants suitable for a higher temperature environment requires special skills typically not found with the typical DIY person and should be left to specialty repair persons. That being said after using grilles for 50 yrs. the burner valves have never been a problem. The grille burners, flame bars gates etc fall apart first.
Hope this helps.

Check the orifice
 

Check the orifice where the gas comes out to the burner. Sometimes debris gets in the opening and reduces or blocks the gas flow.

I have had a ptopane grill for many many yesrs and have NEVER had a problem. And when it is worn out i repurpous it, i use ir as a planter.

you may have spiders in the tubs...

Could be an air lock in the line between the tank and grill switches. This can sometimes happen when the gas is turned on at the grill switch(s) before opening the tank valve. Always open the tank valve BEFORE turning on the switches that allow gas to flow to the burners. Hope this helps😎

:boom:

:boom:

:boom:

spider web in line, perhaps?

///