The chain under load, pulling the bike, will be tight on the top run and looser on the bottom run. That's why, if you're using a tensioner, it needs to be on the bottom chain run.
You're looking for 1/2 to 3/4" slack in the chain. To measure this, roll the bike forward with the clutch engaged, against the compression stroke. The top chain run will go slack. That's where you measure the slack. Adjust your tensioner, or move the rear wheel toward the rear in the dropouts until you get that 1/2 to 3/4"
Chains that are too loose or too tight can cause problems especially if your chain alignment is off.
Make sure your rear sprocket is centered on the hub so there is no up/down movement which will loosen then tighten the chain. The sprocket MUST be concentric with the rear hub.
If you're using the kit supplied #415 chain you might want to replace it with an industrial grade #41 chain. They are usually available at industrial supply outlets such as Grainger, Tractor Supply and even some Ace Hardwares.
Tom