Pedals no longer increase speed after a relatively low speed
The gear ratio you're using is determined by the ratio of the number of teeth of the chainring (gear in the front) and cog on the cassette (gear in the back). For a fixed cadence (number of times you turn the pedals per minute), the higher the gear ratio, the faster you go.

So, you need to either shift into a higher gear (move to a bigger chainring and/or a smaller cog on the cassette), or pedal faster.

Note that most bicycles have the ability to freewheel, which means that if you're not turning the pedals fast enough, the bicycle basically coasts. Contrast this with a fixed gear bicycle, which so long as the rear wheel is turning, the pedals will be turning. If you're not pedaling fast enough with a bicycle that can freewheel (like yours), you're basically coasting. So, if you want to use that gear, you need to pedal faster in order to get power through. And if you're maxing out the highest gear, you may want to change the cassette/freewheel to get a smaller smallest cog in the back. 

If your cadence is below 60 rpm, chances are it's too slow -- you should probably aim for around 80 rpm.