too low extention time for Taq polymerase PCR
To get amplification of a DNA fragment you need primers to anneal first so that you can extend from. The extension time is designed so that DNA is synthesized from one priming site to the other. Therefore if you have a lower extension time you will make a product that is missing the other priming site. This means that all new template strands can be amplified from one end only and you get linear amplification, not exponential. Therefore there will be too little DNA to visualize on a gel.