Story time!

I made a command block with this command:

/summon MinecartSpawner ~ ~1 ~ {SpawnCount:7,SpawnRange:2,Delay:1,MinSpawnDelay:100,MaxSpawnDelay:100,MaxNearbyEntities:100,RequiredPlayerRange:10000,SpawnPotentials:[{Type:"Slime",Weight:1,Properties:{Size:1}},{Type:"Slime",Weight:2,Properties:{Size:0}},{Type:"LavaSlime",Weight:1,Properties:{Size:1}},{Type:"LavaSlime",Weight:2,Properties:{Size:0}}]}

It didn't spawn anything. Ever. Not when it was on solid ground or up in the air, not when it was day or night or high in the sky or low on the ground. Not when it was on stone or mycelium and not when it was in Plains, of all biomes.

After some experimentation, I discovered that if an EntityId tag is provided, the spawner will ignore any and all SpawnPotentials tags and just spawn the entity specified in EntityId according to the other tags. If I remove entries in SpawnPotentials such that only 1 Type (Like only Slimes or only Zombies, but not Slimes and Zombies) of entity is listed in SpawnPotentials, it works as expected, however it still sits around awhile with the Delay tag reading 0. (Like if the Delay starts at 0, it will wait a second or two before spawning, then it proceeds as normal).