Is there gas in your tank(s) and did you sloowly open the valves?
If you don't have an auto change regulator, did you switch it to the full tank?
Did you turn on your tanks, then light a stove burner for a minute or two to bleed the air from the lines?
Did you check the reset on the ECO/Hi-Temp Thermostat?
Is there insects or spiders at the orifice or in your burner tube?
Is the igniter/flame sense probes clean and the gap set properly?
If you have a Suburban WH you can download the service manual from the Library section of the forum. Look for WH_Service and Training Manual 5-18-11.pdf in the following link, Plumbing, Propane and Water Systems
Whenever you have a problem and need help, please give the make and model of the appliance.