I'm no electronics expert but I would check for polarity.
Question: do you have the original plug? If so, is it two prongs or three? Most lamps are two. Assuming you have the original plug and it's two prongs, I would bet that one plug prong is bigger than the other. That means your device is polarized. It matters which wire goes where in the outlet.
Look on your lamp cord and you should see that one side is molded differently than the other. One is usually smooth and the other has some sort of texture or ribbing. If you added more wire between the lamp and the plug, make sure that the smooth wire on the lamp is connected to the smooth wire on the plug.
I am guessing that your lamp is polarized and that you mixed it up when you added the extra wire (easy to do).
Now, why would it make a difference? Our household alternating current circuits consist of a 'hot' and a ground. The ground literally is connected to the ground outside your house. The hot comes from the power company and it does all of the pushing and pulling of electrons in the circuit.
Again, I am no expert but I would bet you that the dimmer you have expects that hot wire to be on one side or the other.
What lamp is it? Any make and model? The other thing that might work would be to post images of your lamp on here.
by oldturkey03
Adding more wire is a good solution to your problem, if done correctly. Did you cut the wire between the plug and dimmer, or between the dimmer and lamp?
How many cores did the wire have at the place you cut it? (Lamps usually have 2 or 3 cores in the wire).
How did you join the new wire to the old wire?
by GrandadTom