What Are the Reasons for Concrete Sealer Failure?

There are several reasons for concrete sealer failure which results in peeling, flaking, or discoloration.

Cheap sealer I'm a firm believer in "you get what you pay for". If you pay bottom dollar for a product, then you're going to get a product with poor quality. Avoid sealers that are sold at home improvement stores because they just don't perform vey well and you risk significant discoloration. I recommend contractor supply stores and make sure the label says "non-yellowing". You'll pay a little more but it's well worth it.
Other chemicals or foreign substances not completely removed before application This is the biggest cause of concrete sealer failure. Anything, whether it's cleaners, dirt, oil, etc., that doesn't get removed from the surface will prevent good bonding between the sealer and the concrete resulting in peeling or flaking. Always test the surface by wetting it and looking for water beading as this is an indication that more cleaning or rinsing needs to be done.
Sealer applied before concrete dries completely Moisture also prevents good bonding resulting in flaking. Sealing over moist concrete can also cause cloudy or discolored spots. Make sure concrete is completely dry before application.
Wrong type of sealer used Certain types of sealers don't perform well in certain applications. For example, epoxy sealers shouldn't be applied to exterior surfaces because they don't allow the concrete to "breathe". Always read labels before deciding which sealer to use.
Surface not neutralized If you've done any acid etching or acid staining, you must neutralize the surface before sealing. Sealer will not bond well with an acidic surface. The easiest way to do this is with baking soda and water.
Sealer applied too heavy This has happened to me before. I applied a heavy coat of sealer so I would only have to do it once. It took too long to dry and the hot sun blistered it. Remember that several thin coats is always better than one heavy coat.

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What Are the Reasons for Concrete Sealer Failure?