Secondary imperfective

Question

Hi,
today, while working on my thesis (on occasionalisms in the poetry of Marina Cvetaeva), I stumbled upon something strange.
My theoretical grammar tells me that a secondary imperfective might sometimes involve an accented 'o' in the stem to be changed into a an accented 'a' (for example: вздрóгнуть --> вздрáгивать). However, I noticed that sometimes this 'o' doesn't need to be accented in order to change upon secondary imperfectivation (for example: всмотрéться --> всмáтриваться). This is also reflected in Cvetaeva's use of secondary imperfectivation (for example, she uses горéть to create погáрывать). She does this consistently; it is not an one time occurence.
So I wonder, is there perhaps another rule? Or is the rule that my grammar book supplies just narrow? I tried finding some articles on the matter, but it's like looking for a needle in a haystack.

Thanks in advance,
Kelly Willersen, Student at the University of Amsterdam