Why is roof buckling?
If the roof sheathing was replaced the spacing of the roof sheathing is most likely the issue.

You can check this by going into your attic and checking that there is spacing between the roof sheathing boards (likely OSB plywood)  board to board typically 1/16 , 1/8 to 3/16 of an inch spacing which allows for expansion, this distance varies depending on direction [top-bottom Long side to long side , width side to Width side].  The board might have an installation instruction stamped on it - that would give you proper spacing for those boards. If the board was installed with those instructions on the attic side of the board you will be able to see the directions for spacing of those specific boards from inside your attic  (if all are buckling or only a few - check the ones that are buckling and the ones closest to them). 

After verifying the information above - contact another roofer mention your info and get a quote for a repair and his written explanation of what caused it. 

After you have the quote; write the original roofer requesting he repair the roof as it was not installed properly (be cordial and let them know another roofer has looked at the issue as well) allow him time to respond 2 weeks is more than enough time. 

During this time you can also get a free consultation from an attorney take your info with you, then if your roofer does not respond let the attorney deal with it so you can have another roofer fix it. 

Of course sometimes all the original roofer needs is a letter from an attorney on the attorneys letterhead.