Boldface won't hold

Using Word from Office 2000 in XP.
Sometimes, when I boldface a word or sentence by selecting it and then
control B it or hit the dark B on the menu bar, it reverts from bold back to
normal as soon as I set the cursor in another paragraph, then I have to go
back and do it over again. After the second time, it always holds. Hint:
when this happens, a square bullet point formatting mark appears in the left
margin of the line where this is occuring. This is in View/Print Layout,
where I normally stay, and all the paragraph marks, etc. show.