This may sound very obvious, but was (a) the watercolour paper dry when you applied the Masking Fluid, or (b) was the painted Watercolour paper damp, or not quite dry when you rubbed off the masking Fluid?
I apologise for stating the obvious, but it is worth stating for anyone else reading this reply.
generally, I would shake the bottle of Masking Fluid prior to using it, as its slightly thixotropic, and so by shaking it, the flow is improved.
I've rubbed the surface of watercolour paper before today because I've been impatient to get to the next stage, but these days I wait until the surface is totally dried, also to the extent of ensuring that the paper is not kept in a damp, cold place when its drying.
If there is something different from the above, then come back to me and we'll try to get to the bottom of the problem.
Very best wishes,
RSC Wigan / Warrington area