向下滚动时，UITableview不可见最后一个单元格

I'm very new to iOS. I have a UITableView that filled with many custom cells, but the bottom cell is is not visible properly when scroll down.my Y value of the UITableView is 44 and Height is 480. Only the half of the last cell is visible when scroll down. How can I fixe this problem.I'm very new to iOS. I have a UITableView that