Couplers are supposed to fit snugly to prevent leaks, but liquids and gases have a way of sneaking past and seeping out. Occasionally this is due to real damage, but a lot of the time, it's a result of a minor misfit or simply age. If you've just placed a new coupler on a couple of pipes and are seeing leaks, the following could be the reasons why.
If the coupler threads onto the pipe, much like a showerhead does onto that pipe that sticks out of the shower wall, then small spaces around the threads could allow gases and liquids to seep out. The answer here is to fill in those spaces. You can use plumber's tape, especially if the pipes are carrying water, but if you're fixing more than just a showerhead, also use a thin layer of pipe joint compound on top of the tape. That compoud adds just a little more protection for connections that will be hidden away.
Of course, if the pipes are carrying anything other than water, you'll want to be sure the material in the pipes won't react chemically with the tape or compound. But in most cases, just adding those fillers will help stop the problem.
A coupler that fits snugly should be the right size, but sometimes "snug" is a relative term. What you might think is snug is actually the wrong coupler and the wrong size. This is more common than you might think in DIY projects, so if you're attempting to connect pipes yourself in a task at home, don't worry. Measure the diameters of the inner and outer circles formed by the pipes you're trying to connect and head back to the hardware store.
Deterioration With Use
The coupler may be new, but if the pipe is older, the ends may have deteriorated a bit with repeated coupler additions and removals. If you can replace the pipe itself, that would be a good idea because the deterioration may only continue. Using something like plumber's tape would be only a temporary fix that could fail if the pipe is weak.
If you need to find new couplers and double-check pipe sizes, look for a specialty seller who carries industrial supplies. The better the selection you have access to, the easier it will be to find the couplers you need to reduce the chances of encountering leaks.