Working Through Some Technique Issues

So, this weekend I spent my time finishing an award-scroll that will be given out next weekend, I'll post details at a later date once I know it's safely in the recipients hands. But recently I've been having some issues with my works and now that the same thing has happened for a third time its obviously a pattern. What do they say? First time is a mistake, second time is bad luck, third time is a pattern.

So here's the issue, I'm having a problem with mediums not "sticking" to my pergamenata surfaces.

My first reaction was that I'd mixed my powdered pigments into paint that was too brittle. This is completely plausible and I'm sure it is definitely part of the problem since I'm pretty new at this. I've been creating test patches for the batches I've made and they have stuck to the test-strips with no issues, and seem to be right as the colour hasn't transferred off when I've wiped paper over them. I have been following the recommendations of others although as yet I haven't added any honey / sugar into the mix for more flexibility. I will try this on my next piece as I'm sure it will probably help.

The reason I feel that this is only part of the issue is because I have also experienced the same chipping / flaking when I've used commercially made Windsor & Newton gouaches.

I'm beginning to think that my working surface is also part of the problem. Everything I have read says that pergamanata DOES NOT need any surface preparation before use, you can just go ahead and start working on it. However, I've been finding what I can only call "oily" areas dotted around the surface, not everywhere, just small patches here and there where the paint will pull back from where I've applied it. Initially I thought I must be transferring oils from my hands onto the surface so I started wearing gloves and covering areas I wasn't working on with glassine-paper. This made no difference, I still came across these small patches.

I'm wondering if it might be worth putting a little bit pf prep. work into the surface of my page. Maybe a light sanding with a VERY fine-grit sandpaper to create some "tooth" for the paint to adhere to? Maybe a light coating of dental pumice to remove these small oily spots? Has anyone experienced these issues before and can offer any suggestions? I really love the look and feel of perg. as a working surface and I really want to remedy these issues.