On a standard switch start without capacitor circuit involving choke, one lamp and one starter, if all components are matched with appropriate wattage, and supplied with correct line voltage, what could make the starter not shut down once the tube is lit ? I have a fixture with a 15 watt choke, FS-2 starter and F15T8 lamp. This fixture is used almost every day, sometimes with 2-3 or more starts per day, with a total of about 3 hours use everyday. I have been using it for about a year now. There is no end blackening on the tube, and sometimes there is perceptible flicker when lit. I suspected a faulty starter, replaced it with a green-cased Leviton FS-2 and now when it starts, sometimes the starter stays lit, and the tube rectifies. I reinserted the tube end-for-end, and it seems better, but occasionnaly mis-fires. When rectifying, usually it only lasts about 1 second, but sometimes can be as much as 5-10 seconds, causing the starter to close again. Any thoughts as to why this is happening ? Thanks in advance.

P.-S. I am in 120V 60Hz territory.

There may be bad contact (arcing) somewhere in series with the lamp. The fault might even be inside the lamp

The ballast might be of the wrong power

On a standard switch start without capacitor circuit involving choke, one lamp and one starter, if all components are matched with appropriate wattage

Even when this is not the problem with your lamp, I have to warn: The lamp wattage alone is not relevant, the matching should be done based on rated ballast/lamp/starter types. E.g. starter labeled "4..25W" won't work with F21T5, in he similar way e.g. 230V ballast rated "18/20/24/26W" would overload the F21T5 lamp.
Technically for the starter is important the lamp arc voltage (should be lower) and ballast OCV (should be higher), for the ballast the lamp operating current (should match the lamp rating) and sufficient OCV (generally at least twice the arc voltage)

and supplied with correct line voltage, what could make the starter not shut down once the tube is lit ? I have a fixture with a 15 watt choke, FS-2 starter and F15T8 lamp. This fixture is used almost every day, sometimes with 2-3 or more starts per day, with a total of about 3 hours use everyday. I have been using it for about a year now. There is no end blackening on the tube, and sometimes there is perceptible flicker when lit. I suspected a faulty starter, replaced it with a green-cased Leviton FS-2 and now when it starts, sometimes the starter stays lit, and the tube rectifies. I reinserted the tube end-for-end, and it seems better, but occasionnaly mis-fires. When rectifying, usually it only lasts about 1 second, but sometimes can be as much as 5-10 seconds, causing the starter to close again. Any thoughts as to why this is happening ? Thanks in advance.

P.-S. I am in 120V 60Hz territory.

Now back to your case:
- The lamp may have worn out cathode emission layer. This does not have to be accompanied by the blackening, mainly if it is not normal wear, but some fault (e.g. the coat cracked and had fallen off the filament). The blackening start, when the lead in wires or the tungsten filament core sputter, but that may be prevented when there is still some emission layer on the electrode, witch could take over the discharge. And when the layer is not enough anymore, the lamp start to rectify and/or the starter trigger.
Check another lamp (good one) and if the problem disappear, take lamp current and voltage measurement (to double check they are correct)
- Some loose contact (as noted by Ash)

I have had this problem before on an F15T8 preheat fixture, and it turned out to be the tube (cathode inside)