[NEST Users] Simple Additive STDP Dynamics (`stdp_synapse`) only seeing LTD not LTP

Hi,

I'm attempting to recreate the STDP dynamic figure like that of [this figure](https://www.jneurosci.org/content/jneuro/20/23/8812/F1.large.jpg?width=800&a...) (from [this paper](https://www.jneurosci.org/content/20/23/8812)).

However, no matter how I tune the time-constants and transmission delays, it seems that the potentiation side of the dynamics is just missing. I've checked the NEST ML equations and C++ header files and they look fine.

Thus, I am needing a second opinion to see if I am missing something obvious. [Here](https://gist.github.com/aoot/9409b939e891a1c823cc7defe97ab479) is the code in ipynb form and the figure I have (missing the LTP side).

Thank you, Anthony