Hi,
Using that formula, the hysteresis voltage would be:
Vhys=Vcc*R1/R2
and with R1=1M and R2=3M and Vcc=15 we get:
Vhys=15*1/3=5v
so the low trip point is Vcc/2-Vhys/2=7.5-2.5=5v and the top trip point is 7.5+2.5=10v.
This means the inputs do not get recognized as a logic high until the input reaches 10v.
After that, a low will not be recognized until it reached down to 5v.
So to see any change an input would have to go to around 11v and down to around 4v to make sure.
That is assuming that the output goes all the way to zero and all the way to 15v when it switches.
Try again and see what happens. It also may not work as well like this with only one feedback path, we might have to use two separate Schmitt Triggers on the input for correct operation to keep the trigger action independent for each input channel.
The purpose of Schmitt Trigger inputs is to help prevent false triggering from slowly increasing or decreasing signals. A slow signal on a regular gate could make the input seem ambiguous to the gate and thus cause the output to jump up and down while it moves through the threshold region. The hysteresis means that once it switches, it stays at that logic state until the input again changes more drastically which would normally mean a real signal not just noise.