Did you use the correct product according to the directions?
What are you needing to clean off of the deck?
Here in the S.E. mildew is the main problem so I usually just use bleach [mixed 50/50 with water. I'll wet the deck and then spray on the bleach/water solution with a pump up garden sprayer, let it set but not dry and then rinse with the PWer. Stubborn spots can be taken care of with a 2nd application and/or maybe a little scrubbing.
almost forgot

Welcome to the forums!