If you get a positive result (meaning a baby has DS or one of the many other trisomies) that they test for then this is 100% that the baby has it. If on the other hand you get a "no abnormality detected" it means just that - no abnormality detected - it does not mean 100% that your baby will not have a trisomy - it means that it is extremely unlikely that it will have them that's why they use "no abnormality detected" - to cover themselves just in case something in the test did not work. Extremely rare to have that happen, but it can happen.
The possibility of this occuring is minute. But here is an explanation of how it could theoretically happen.

CVS take a sample of cells from the chorionic villus (the finger like projections) from the placenta and examines a number of the cells to see the chromosomes. The original cell (that creates the baby) splits and splits again and again to produce the placenta and baby.

For a baby to have Down syndrome, and CVS to show no cells containing three copies of chromosome 21 (which causes DS), a mutation must have occurred only in the cells that went to produce the baby (and not in the placenta).

If this occurred, it would also be possible for only some of the cells in the baby to have three #21s, and depending on their location would influence the 'level' of Down syndrome.

Amnio takes a sample of fluid surrounding the baby which contains cells that have been released from the baby. These cells are closer (in age and arrangement) to those in the baby that cells from CVS, and thus are a more accurate representation of the baby.

If the baby is formed by some cells that have two #21s, and some cells that have three #21s, and the amnio sample only contained released cells with two #21s, the result would come back as "normal". For the sample to contain only cells with two #21s, it is more likely that there is a higher ratio of these cells to those with 3 #21s in the baby.

The chance of a CVS or amnio reporting a false 'normal' result is extremely tiny, but i guess the possibility is still there.

k