Minecart wont travel up hill past level 28.

Minecart wont travel uphill past level 28. The mine cart will travel up to this point from bed rock,then go back down. You can go 3 blocks above this level and the cart will go all the way up, with no problems.

The problem seems to be around level 28 from bed rock. It works great up to this level then stops like something in it's way.
Try to replace blocks with other blocks no change. Something stops the cart at this level only.