Weird problem with an LM339

So I have an odd issue.

I have an IR sensor that feeds between 0.3V-5V into an LM339, using 3.3V as the comparison voltage.

But regardless of what the input voltage is, the output is always low.

I have 3 other IR sensors, and they all work. I've checked every voltage, and the IR sensor in question is behaving normally. All other 339 outputs for the other IR sensors are fine. I swapped out the 339 for another one, and same problem.

My first move was that there was something wrong with the pull-up resistor, but all the connections seem fine too.

I have no idea what's wrong here - any ideas?

Did you put a pullup on the output of the comparator, I think it is open collector?

Edit...Oh wait, you did mention you have a pull up.

I know you said the IR sensor is acting normal but try swapping with one of the other sensors and see if the problem is the sensor.

It's bias and pull-up problem only. send the circuit for analysis.