Is your Hamamatsu L6299 UV light underexposing or not turning on at all? If you have checked the lamp and igniter, it is very likely a problem with your lamp power unit. This is part number AG09.  These boards are taken from fully functional DNS tracks at GetSpares.com.  To test yourself, all you need is a high power 10 ohm resistor.  Note that any time you are working with power supply boards, there are high voltages that can be deadly.  If you are not comfortable working around voltages as high as 400V do not try any of these tests.

Check HV Tabs Loaded and Unloaded. Also make sure input power is ok.

First thing to check is that the open circuit voltage is between 50 and 55V. Open circuit means that everything is disconnected from the HV+ and HV- tabs as illustrated above. If it is not withing this voltage, there is a problem with the board and it should be repaired. If it passes this test, the next step is to load it with the 10 ohm resistor.

With 10 ohms connected to the HV+ and the HV- tabs, the voltage should be no less than 33V across the resistor. If it is lower than that, there is a problem with the board and it should be repaired.