Spawned Entities not being detected by @e parameter

When I run a command block the the command "/execute @e[type=Slime] ~ ~ ~ /scoreboard players add @e[type=Slime] test 1" the command blocks will not detect the entity if it summoned with command or spawn eggs until after I close the world and relaunch it. Occasionally the game will create a non-player ID with the selector "@e[type=Slime] and use that as a scoreboard value.