some questions with the IBIS models

Hi All:
This is my second time to pose a question in the Si list. I just start
working as a Si engineer a few weeks ago
,and the problems that I am facing right now is trying to verifly the
IBIS model.
The procedure that I am following is like this:
1.use ibis2xtk to transfer the file from ibis to QUAD
2. then I use the scratchpad to build the schematic according to the
spec for the R-fix and Voltage
(i.e. I am building the six files for XNS)
3. then I use XNS to observe the waveform and check the rising and
falling waveform

My problem is even though I am building my schematic use scratchpad
according to the spec,
but my waveform can not acheive its full voltage swing . I wanted to
know why , and The way that
I should approach to fix this problem.

thanks to all of you that they can help to solve this problem as this is
very important for me to get this concept

thanks
Jason Leung ( Alcatel)