make sure your properties allow for leash bounds that are not too big or else your critters may spawn inside geometry and you wont be able to see them. you can test the size of leash bounds by double clicking on your spawner and in the 3d data tab type in a number value for test radius, it will render a blue sphere around your spawner to let you know exactly how big the radius will be.
secondly, remember, it is a activator object. and if you have ever messed with mannequins or weapon racks you may know that all non-3d activators have some bizarre quirk where they dont work properly if set at square angles. try rotating the spawner on its z axis at an offset with a random decimal value. (i usually keep all my activators at a z angle of 94.15 degrees)
last, check to see if your critter is nocturnal or whatever and make sure you are visiting your cell with the spawner at the correct time of day.
other things to remember
fill out the properties properly. make sure the formlist is the right one for the spawner type. a "bee spawner" will not automatically spawn bees on its own, you may have to select the formlist manually to select your spawner to spit out bees