It is possible that the deck wood was sealed too soon after being constructed. New cedar wood needs to age or weather for at least 6 months or more to remove the mill glaize material and oils from the lumber mill. This mill glaize can prevent the sealer from soaking in effectively. Also this weathering allows the sap and tannens in the wood itself to leach out and escape and allow a sealer to soak into the wood.
Consumer expectations around these longer-term sealers is a very hot topic. There are consumer expectations from “over-selling” and over-promising the performance of these sealers that are not within the ability or design of these sealers. These penetrating sealers are not designed to prevent splitting, checking or cupping. If your wood dries very fast, usually in direct sunlight, then the wood is prone to split, check and cup. A good sealer can lessen or slow this damage but will not stop it from occurring.
The location of your wood deck contributes to this problem. A wood deck on the southern exposure of a house is more prone to cracking as it gets that pounding, direct sun all day and will have more warping problems than a deck on the shady north side of a house. This prolonged and intense sunlight exposure will damage and shorten the life of a wood deck just as it would damage our own human skin. We can slow the effects with a sealer but we cannot stop the effects of Mother Nature.
Sealers will not be able to stop the cracking and splitting or your wood deck, especially if the material is treated lumber which is actually treated yellow pine. Yellow pine is a very strong, tough wood, used for structures and is designed to last a long time. This yellow pine species of wood is not a fine or pretty wood material and it is intended and commonly used for framing lumber. It is however a lower cost material and is commonly used on wood decks.
I suspect that your deck is behaving just as it is intended, and that your sealer is effectively protecting your deck as it was formulated. This sealer, or any other sealer, will not prevent the prolonged and savage effects of intense direct sunlight.