A couple of thoughts:
Check your solder joints and such for connectivity.
Some ws2812b LED strips need 5v to light up, many only need 3.3v to light up. Now, since you didn't mention powering the strip from the Teensy (which you can do for a few, but doing lots of LEDs can easily run into limits of the power supply), I assume you are powering it externally. What does your external power setup look like?
If you are using a USB battery, many batteries will shut off if the power drain is too small. It may be that lighting up just one LED is not enough of a power draw to keep the battery active.
If you are powering the strip with 5v, it may be that the strip is not seeing the transition on the Teensy data pin from high to low or low to high. This is typically because most hardware will only see the data transition if it is at least 70% of the powering circuit. 70% of 5v is 3.5v, and the Teensy 3.6 only emits 3.3v. So some strips will see it, some won't.
You might want to try first using the Adafruit_Neopixel library, as it is simpler than FastLed, just to make sure the connection is setup.
Again to debug the basic connection, try wiring up the strip so that 3.3v from the Teensy is sent to power the strip. Note, as I said, some strips can work with 3.3v (typically the older LEDs and the newest ones). The 3.6 can provide something like 250mA, which is ok for 3-4 LEDs, depending on how the Teensy itself is powered. If you turn down the max power of the light, you can run more. You can try powering the LEDs with the VIN pin (5v if USB), but you have to be careful, since the Teensy 3.6 cannot handle 5v input.
If your strips need to run at 5v, you probably need to get a 74HCT245 or 74HCT125 chip, and hook that between the Teensy and the LEDs. Most voltage converters that you find are not fast enough to drive WS2812B LEDs, but the 74HCT245 is.
If you are going to run a lot of strips eventually, you probably want the Teensy Octows2811 from PJRC, that handles all of the voltage conversions, etc:
In the future, it would be helpful if you also had a picture that showed how all of the wiring is done. I've certainly mis-counted in the past and hooked the wire to pin 4 when the code is using pin 3.
If you haven't done so already, you might want to read the Adafruit Neopixel best practices guide (Neopixel is the name Adafruit uses for its WS2812B and SK6812 LED strings/rings/matrices/etc.):