The most common reason for this (at least in my dealings with siFR) is that the replaced CSS style that lies "behind" the siFR rendered style is too small. sIFR applies its own style rules, but reserves space for the rendered font according to that underlying style. If the total width of the text in that CSS style is not as wide than the text rendered by SIFR, SIFR assumes it has run out of space and makes a line break.
Try giving that style a larger letter-spacing
or padding
. Often, this is caused by a negative letter-spacing
that you just need to set back to 0.
Remember, I am not talking about the style you define for the siFR font in the settings file, but the one you are replacing!