I can't increase or decrease before and after paragraph to a little extent

Hi There,

I am new to InDesign, so learning it's features one by one. I have a manuscript (created by another designer), in which there are mutiple returns in between paragraphs instead using before and after spacing. Now I want to remove those multiple returns and use before and after spacing. But surprisingly found that I can't increase or decrease the before or after spacing to little extent. As for example if I increase them gradually, before putting 6pt (I want even 3pt spacing), no spacing is inserting between paragraphs. Then if I increase the number gradually no changes in spacing can be seen until 12pt. So if I increase it to, let's say, 8pt, the spacing is not increasing more than 6pt. It only becomes double if I increase it to 12pt. But before 12pt, no more increment in spacing.

If I create a new indesign document and create paragraphs, then even very little before and after spacing can be added gradually. Then why this is not happening for that specific indesign file. Is there any settings that is responsible for minimum (here 6pt) increment/decrement for the before and after spacing?

Thankyou