A pond has everything necessary to support algae and weed growth: plenty of water, sunlight, and nutrients. Some ponds offer more sunlight and nutrients than other ponds, causing the fight against them to be difficult.
Just because there is something in the pond doesn't necessarily mean it is live growth! The best indicator of this is the color. Dead growth will be brown, while new growth will be green. If you're seeing green growth, make sure it is correctly identified before treatment. The type of growth will determine what type of product you may need! If you're seeing brown algae after a treatment, it could take a little time for that to "go away" and drop to the bottom of your pond. A good, hard rain should do the trick! If no rain is on the radar, then spraying it down with a hose should help break it up and drop it to the bottom.
Keep in mind that all dead vegetation and debris that comes into your pond will form into muck/sludge over time. Not only does this buildup eventually decrease the depth of your pond, but it provides nutrients for more growth in the future. For these reasons alone, it is important to stay on top of your muck maintenance!