If batch of home brewed beer isn't fermenting can you just add more yeast?

You can add more yeast to a stuck fermentation, but there are a few things you should consider first:

Wait for one day before determining whether your ferment is stalled or not. Some brews are very slow and releases tiny, difficult to spot bubbles whilst others will ferment like crazy. A sure fire way to check if a ferment has stalled is to siphon some of the liquid into a plastic bottle and screw the lid on tight. Leave it for about an hour (in the sink or bath tub) and if the bottle expands, your yeast is active.

If you are certain your yeast is stalled. Try to identify why this has happened before adding any more, if conditions aren't right you'll only be sending more yeast to their death which can ruin the flavor in the long run.

Different yeasts have different temperature thresholds, too hot or too cold could kill the yeast or put it to sleep. Check the conditions your yeast works best in and, if appropriate, try to relocate your carboy/brew bin to a warmer or cooler location before adding more.

Certain brewing additives can also kill your yeast, such as sodium metabisulphate (present in Campden Tablets, stabilisers and some sterilizing agents) - if you have used any additives, make sure twenty four hours have passed before doing anything else.

Once you have gone through the above and are certain you have done your due diligence in trouble shooting - you can add more yeast. The best way to do this is through the 'starter bottle' method. Siphon some of the liquid into a small glass or bottle, add your yeast (following any instructions required to activate it) and wait. Once the contents of the glass or bottle have started to foam, you know that yeast is active. Go ahead and tip it back into your carboy or brew bin.