Why can I not use the bottle again? The nob on the end turns on and off just fine.

Thank you for submitting your question. Spray & Forget gallon concentrate is a different formulation from the hose end product, and you would not get the right concentration in the hose end container to achieve optimal performance - thus, the hose end product is not refillable.

Thank you for contacting us. A refill product is not available for the Spray & Forget Super Concentrated Roof Cleaner Hose End product. The gallon container is not a refill for Spray & Forget Super Concentrated Roof Cleaner Hose End as it is a different formulation from the hose end product, and you would not get the right concentration in the hose end container to achieve optimal performance.