That shouldn't happen. Its quite strange. Can you please provide your PSoC Creator project? It will make debugging easier.
Its similar with code example (ADC_single_ended). Only change is ADC Resolution of 8 bit.
/* Start the components */
/* Start the ADC conversion */
/* Display the value of ADC output on LCD */
output = ADC_DelSig_1_GetResult8();
This function ADC_DelSig_1_GetResult8 returns the result of an 8-bit conversion. If the resolution is set greater than 8-bits, the LSB of the result will be returned. When the ADC is configured for 8-bit single ended mode, the ADC_GetResult16() function should be used instead. This function returns only signed 8-bit values. The maximum positive signed 8-bit value is 127, but in singled ended 8-bit mode, the maximum positive value is 255.
Just change ADC_DelSig_1_GetResult8 to ADC_DelSig_1_GetResult16 and it will work.