What are the possible causes when the output voltage of CMOS logic ICs does not rise to the power supply voltage?

The main reasons why the output voltage of CMOS logic IC does not reach the power supply voltage are considered as follows.

1. A load exceeding the drive capability of the product might be connected.
   Check the output current (driving capability) of the buffer and the driven load.
   If the drive capability is insufficient, consider using a product with higher output current.
2. The input signal frequency might exceed the maximum operating frequency of a buffer.
   Check if the input signal frequency exceeds the maximum operating frequency of the buffer. If so, consider using a faster buffer. Logic ICs with relatively high operating speed include the 5 V VHC series (59 MHz in the case of the 74VHC244), 3 V VCX series (114 MHz in the case of the 74VCX244), and 3 V LCX series (77 MHz in the case of the 74LCX244). 

Please refer to this section for the concept of maximum operating frequency.
FAQ: How can I find the maximum operating frequency of a general-purpose logic IC?