[GeoffR]

One problem is that the PANOSE weight settings in the font are incorrect. Use fontforge to change the PANOSE weight of the -Bold and -BoldItalic fonts from Book to Bold should fix it.

Edit: There is a second problem: The TTF names in the -BoldItalic font are incorrect. Use fontforge to change the Styles (subfamily) from Italic to BoldItalic should fix it.

Edit2: A more recent version of the Merriweather font, version 1.570, has both of the above faults plus a third: The family name of the -BoldItalic font is incorrect. Using fontforge to change PS Names> Family name from from "Merriweather Bold" to "Merriweather" fixes it.