Trouble Detecting Watermarks On Older GB Stamps

I'm having a little difficulty as I try to catalog some of my Queen's head stamps from Great Britain during the 50's. These stamps were issued a number of times in different years and the main discerning quality among them seems to be different watermarks. However, I am having no luck whatsoever when I apply my watermark fluid to them. I have them face down in my black tray, but there's just nothing to be seen. Is this a common problem?

I have a similar problem with Romania,
really, quite clear watermarks but the process needs a tweak.

I use the bottom of my black tray, there is a little pool /dent
where I squeeze in the fluid, drop the stamp in it, the shift around
with the tweezers, whilst shining a Dolphin Lamp / Torch on it,
at an angle.
The mark suddenly comes into view.