need more clues
We need more clues to solve your problem. Here are some tests to do and report on.
0. Double check the digital number format for what you expect.
1. Are all of the power pins at the right voltage?
2. Is the clock waveform the proper voltage levels and frequency?
3. Put 0V on the analog input and see what the digital output is. Is it what you expect, is it a constant value each conversion time or does it jump around?
4. Put half of full scale DC voltage on the input. Is the digital output what you expect?
5. Apply a very low frequency sine wave on the input and see if the bits toggle. Higher order bits should toggle less frequetly than lower order bits.
If at all possible, put the digital output to a DAC and examine the waveform.