I installed an 18' x 23' uncovered mahogany deck last summer. The deck is attached to the house with a ledger board and sits on posts on concrete piers. Frame work is PT; the decking is tongue-and-groove mahogany. My problem is that some of the decking is buckling significantly (>1"). The first 4 courses of mahogany are screwed down with SS deck screws. The rest of the mahogany is blind-nailed with port-a-nail, SS, serrated flooring nails. The last course of mahogany is screwed. The mahogany was sanded and treated with clear Behr Premium Deck Sealer which seemed to function well for about 2 months then started fading badly. I suspect that the sealant faded and the wood is getting wet, swelling, and since the last course is fixed, it can only buckle to relieve stress. Comments, suggestions on how to fix this?

PS - I live in Massachusetts and the deck exposure is northwest. The reason for mahogany is someone gave it to me.

A tongue-and-groove deck fully exposed to the weather is always going to buckle. Wood expands and contracts, and there is nowhere for it to go when it expands except to buckle. That is one reason why most decks are built with spaced deck boards.

Thanks for the feedback. My hope was that with the deck sealed well the wood wouldn't swell too much. I'm planning on treating the wood again but with Woodzotic from Restore-a-Deck. I'm hoping this will minimize expansion in the future. I've resigned myself to the fact that I may have to re-lay the deck but with some spacing, say, every 4 courses or so. Does this sound like a reasonable plan? I'm tied into the mahogany since I can't afford to buy new material.

Many factors
1st Richard is correct wood will grow and shrink with weather.
It was inportant when installing the deck that the contractor check the moisture level of the wood to be sure that the wood was not to dry when installed. Not to wet either.
2nd fastening of the deck is critical. New pressure treated lumber requires nais or screws that will not corroid because of the chemicals used in the treated lumber. This will cause poor fastening and allow the wood to cup and lift.
In order to prevent this from happening the contractor should have routed out grooves on the backside of the boards to prevent them from cupping.

You now need to remove the deck material and start again. As sanding the top smooth will not do any good as once the wood dries up, it will crack.

All of the wood should have been treated with at least two coats of waterproofing on all sides and ends before it ever got installed. Treating just the top side is going to buckle because of one side being dry and the other being wet.
If the deck had of been built without the tongue and groving using tiger claws to hold in place, and the wood was sealed on all sides it would have held up.