There are three chain measurements, any of which could be making your chain a poor fit.
Width: This is the distance between the plates that make up the chain. Hubs with one cog (single speed, coaster brake, internally geared, and track) use 1/8" wide chain. Hubs with multiple cogs (multi-speed freewheels, and cassettes) use 3/32" wide chain.
Length: How long is the chain when layed out in a straight line? Every bike will require a different length chain. Luckily, you can use a chain tool to take links off the chain, or to add links to the chain. To figure out how long the chain needs to be, drape it through the rear triangle and over the cogs. Make sure the wheel is most of the way forward in the dropouts. Grab both ends of the chain and try to complete the loop. If the ends don't meet, add a few links. If the ends overlap, take a few links off so the ends only overlap 1/2 a link to a whole link. Reconnect the ends, and pull the wheel back into the dropouts to set your chain tension.
Pitch: Pitch is the distance between chain pins. 1", also referred to as skiptooth, pitch was common a few decades ago. All new chains and cogs are 1/2" pitch. For simplicity's sake, let's just say 1" and 1/2" equipment is incompatible. Wear is a kind of pitch problem. Chain pins and cog teeth wear down, effectively changing their pitch. A new chain will not completely seat on a heavily used cog. Similarly, a heavily use chain will not completely seat on a new cog. Replace the old component will fix this problem.