The windings are incorrectly connected. You can usually fix this by disconnecting ONE winding and swapping the two wires. If you have a fresh battery connected to your non-working JT, measure the voltage at the collector of the transistor. It should be the same as the battery voltage. If not, you may have the winding wires even more messed up than that (especially if your wires are the same color). Check continuity with a DMM on ohms or continuity range.
The transistor is getting too much bias current and won’t oscillate. If your JT does not work at 1.5V, but works at a lower battery voltage (try a depleted battery), then this could be the problem. Increase the 1000 ohm resistor to a higher value or else add another in series to make 2k ohms.
Often the leads to the transistor are mistakenly reversed. The 2N4401, PN2222A, 2N3904 transistors pinouts are E B C looking at the bottom with the flat side up. The BC337, BC547 transistors are C B E. The Japanese 2SC1815, 2SC945 transistors are E C B.
The 1k ohm resistor should have the colors BROWN, BLACK, RED, GOLD. If not, then you may have the wrong value resistor. The red band is especially important.
The transistor is the wrong type (PNP instead of NPN), or defective. Sometimes the transistor shows no sign of damage even though it has been burned out. Use another transistor. Don’t forget that whatever caused the first one to be damaged might also cause the second (or third) one to also be damaged. It is a wise thing to make sure that there are no other errors before you damage more transistors.
The LED is backwards. Make sure that the cathode (has a flat spot on the plastic next to the lead) is connected to the negative of the battery.
I’ve included a pictorial (simplified drawing) of how a Joule Thief should look. I hope this helps the experimenter get an idea of what he may be doing wrong.
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