Hello,
First of all, are you sure it has met the spawning requirements? Try make sure such as:
1. There is enough spawning space (5x5 at least with spawner in middle) and of course with vertical space.
2. As cow is a passive mob, make sure to cover the floor with grass block (dirt with grass)
3.
Lighting. This is very important as cow is a passive mob, will require enough light around the spawning area for it to spawn. Place some torch near the spawner or on the grass blocks.
4. Stay within 16 blocks spherical radius from the spawner. If there is no player within that radius, the spawner will stop spawning.
After all of those requirements are met, you need to wait at least several minutes before finally the cow spawns. Increasing the spawner level will also increase the spawning and stacking rate.
Tips: At first it will take longer for the first cow to spawn, but once a cow has spawned it will stack much faster. So make sure to leave at least one alive if you are going to grind the cows.
If you tried all above and it is still not working for a long time, try to relog.
If still not working, consider submitting a bug report about this
here.
That's all I can tell, hope it helps, and have a nice day/night!