Measuring node to node capacitance (for Neutralization)

What I want: I have a simple differential pair with PMOS load in an advanced process.

Now I would like to use "Neutralization" to cancel the effect of Cgd. This works by purposely connecting Cn between the gate of the positive side to the drain of the negative side and vice versa. Effectively, this produces a negative differential capacitance which (since it is in parallel to Cgd) removes Cgd.

Now I would expect that my bandwidth even increases (since the coupling is removed) but whatever value I use the bode plot becomes very messy and bandwith decreases. I even tried to connect directly a negative capacitance between gate and drain. Specifically, neither Cgd=2.4f nore Cdg=4.63f (from DC analysis) work.

For this reason I would like to measure the capacitance which is actually there.
I know captab with nodetonode from DC analysis, however, it shows me:

vip: vom   fixed=0, variable=0,  sum=0

vom: ip  fixed=0, variable-0, sum=0

How can that be?

What's even more frustrating is that symmetry on many nodes does not hold, e.g.:

vop : vdd!  fixed=0,  variable=-378.911a,   sum=-378.911a

vdd! : vop   fixed=0, variable=234.089a,   sum=234.089a

How can that be and how can I obtain the actual capacitance values?