When I tried to build a Jump'n'Run parcour I found out that a combination of a command block, a pressure plate and a comperator doesn't activate a command block sometimes. At first the command block worked the right way (it gave me jump boost) but after a while the comperator stopped acitvating. The command in the first block was like that:
/testfor @p[x=-28,y=205,z=66,r=1,score_parcour_min=1]
The parcour-score checks if the player has already started the parcour-challenge. (First picture)
A simular problem appeared when I tried this and combined three command blocks like this:
1.) /testforblock 41 222 35 minecraft:light_weighted_pressure_plate 1
2.) testfor @p[x=41,y=223,z=35,r=2,score_check=2]
3.) /scoreboard players set @p[x=41,y=223,z=35,r=2] check 3
But like the first one the comperator stopped updating after the first run. (Second picture)