comperator won't go off when source is false.

for explanation of the problem we have made an video
https://youtu.be/EQGddbx0rJc

in short, the comparator wont go off in certain situations. an update of the command block before the comparator is required. (and that does not happen)

this is one example route for this problem

• first command block
  /testfor @e[type=Player] {SelectedItemSlot:1}

  repeat - unconditional - always active

• comparator

• second command block
  execute @p ~ ~-1 ~ execute @p ~ ~-1 ~ testforblock ~ ~ ~ minecraft:wool 3
  repeat - unconditional - needs redstone

• comparator

• third command block
  /effect @p minecraft:slowness 1 4
  repeat - unconditional - needs redstone