It happens many times, while performing an experiment the IC output is not correct. So how to troubleshoot a problem. In my experience i perform the following steps while hunting for the troubled area of my circuit layout. Consider the case of optocoupler 6N137. Somehow the output was a continuous straight line at biasing voltage level when given a 100kHz signal. Here is how i solved the issue.

1. I disconnected all the inputs and outputs and thoroughly verified the circuit for any possible shortened paths. The veroboard may have a shortened path. Commercially available verobaords have quality issues and my advise is to use a paper cutter and strike it properly in between the conducting paths before you proceed for the circuit fabrication. A shorted path can also occur as a result of soldering therefore check for the shorted paths using the DMM. 
2. Connect the circuit and see if the input at the input port is OK. Is the input voltage above the working voltage of the device ? If it is ok proceed to the next step.
3. While the circuit is connected see if any of the device is hot. For example in this case one of my optocoupler out of three was hot. Hot means that there is a short circuit somewhere, if there is any and first step is verified then there must be an issue in the IC. 
4. If you are sure that the circuit is fine and was working ok before the problem, one way is to change the IC and check the output. In case the output is ok don't forget to check the grounds of the circuit.
5. Sometimes the IC is ok circuit is ok but you still don't get the output. In this case decrease the switching frequency and see if the output is available or not. For instance if you give a frequency of 100 kHz to optocupler 6N136 it will not give you a perfect waveform. Rather a triangular type waveform will appear. Also verify that the biasing voltage is ok.
6. I had an interesting issue with the DC power supply i was using for biasing. It had an issue with the ground and one time it give me true output and second time no output. Eventually i replaced the DC power supply with a DC charger and the output was ok.