Here are some of the most common reasons bottom paints fail:

Bad surface prep - Paint flaking, delamination, and peeling can happen if the surface wasn't prepped properly, including repairing any gelcoat blisters and making sure the surface is dry, clean, and contaminant-free. Peeling paint on underwater metals happens if the the surface wasn't prepped and cleaned properly, or if the wrong primer was used. 

Paint not applied thick enough - Bottom paint is thicker than topside paint, and shouldn't be thinned just to make it go further. For best results, follow the paint manufacturer's directions regarding film thickness per coat, and be sure to apply the recommended number of coats.

Missing the 'thumbprint tacky' stage when overcoating - If you've applied an epoxy primer/barrier coat under your antifouling paint, you have to apply the antifouling paint when the primer is 'thumbprint tacky'. If you miss this stage, you have to lightly scuff the surface with 80-grit sandpaper or a Scotch-Brite™ pad to achieve a mechanical bond.

Launching before the paint cures completely - Pay close attention to the bottom paint manufacturer's recommended time window for launching the boat after painting. If you launch too soon, the paint won't be cured completely and it won't have the desired antifouling properties or longevity. 

Failure to launch within the recommended time allowed after applying bottom paint - After applying antifouling paint, you may have a limited amount of time to launch before the paint starts to oxidize and lose its antifouling effectiveness. Bottom paint launch windows can vary greatly (from 2 weeks to 18 months). Read the paint label carefully to determine the manufacturer's specified launch window.

Oxidation of the paint film - Trailered boats that are hauled and relaunched repeatedly during the season can be subject to oxidation of the antifouling coating if the bottom dries out. If this happens and you launch the boat, the antifouling biocide can't leach properly, and barnacles can start attaching themselves. If the coating oxidizes, you don't have to repaint. Before relaunching, remove the oxidized surface by scuffing it lightly with a Scotch-Brite pad, exposing a fresh layer of biocide.

Water level is low where boat is anchored - Contaminants can cling to the bottoms of boats lying in the mud at low tide, reducing the effectiveness of the antifouling paint when the tide comes back in. In a low-water situation, it's also possible that the bottom paint can get scraped off by hitting bottom, or by rubbing up against sharp objects stuck in the mud.

Low saline levels in the water - Low salinity can be caused by an influx of fresh water from various sources, such as a river or a heavy rainfall. An antifouling paint's effectiveness in leaching biocide can be reduced by even a temporary drop in saline levels.

Pollutants and contaminants - Contaminants such as chemicals, silt, and other pollutants affect the water's pH balance. If the alkaline level is too high, the biocide can get trapped in the paint, preventing it from leaching. If the acidity level is too high, the acidity can actually destroy the biocide in the paint, leaving nothing to leach out.

Water temperature is too warm/water is still - There is typically more fouling in warmer or still waters than in cooler waters or areas where water flow is unrestricted. If your boat is in an area with still and/or warm waters, check the bottom regularly and gently remove any fouling with a maroon Scotch-Brite pad. It'll also help if you use your boat more often because fresh biocide gets exposed as the boat moves through the water.

Stray electric current or improper grounding - Electric current in the water underneath your boat can actually neutralize the antifouling paint. Sources of electric current include stray electrical current coming from a nearby boat, or improper grounding of the electrical system on your boat. At the end of the season, when you haul out, check your electrical system to see that it's wired properly.