I have some that make a new nest almost every day and then I have some that never make nests. While having a bubble nest is a sign of happiness, not having one is not necessarily a sign of unhappiness or that something is wrong. Some Betta's will only make a few per year. I try not to worry about it. It can also have a lot to do with the type of filter you are using and the amount of current in the tank. High current can actually deter a Betta from building a nest.
You can always tell an unhappy Betta, they are sluggish, sleep alot, are inactive, or exhibit other sicky symptoms like finrot or ich.