I don't think that Ginger Beer has the same proteins that barley/wheat beer does so it possibly won't get the same amount of froth. If there is a big layer of yeast at the bottom, then it may already be finished. You could pour of a very small sample and taste it. If it is sickly sweet, then the fermentation hasn't started. If it is dry, then perhaps it is finished.
21 degrees is on the cool side for a dried yeast. If you can get the temperature up it may help. It also helps if the mix is well oxygenated before adding the yeast. I use a BIG whisk. The yeast use oxygen during the initial process in order to multiply numbers. By the end of that step, there are too many yeast cells for an infection to take hold.
One thing you don't want to do is allow bacteria to get into the brew. If there has been some CO2 being produced and you have a fermentation lock then it should be protected at the moment. Two things to avoid when you have a stuck fermentation at this point are lifting the fermenter by the top (ie handles) and pouring a sample too quickly. Both will suck air into through the air lock and whilst I cant prove it will be a source of infection, it defeats the purpose of the air lock.
Perhaps try warming it up first. Adding some more yeast may help if the fermentation is stuck, but get it started in a cup of body temp water with a 1/2 teaspoon of sugar. Wait until it is foaming well. Yeast don't like big changes in temperature either as it reduces their vigour.