Have you aged your precipitate? It should stay at higher temperature (80/90 deg C) for at least an hour before filtering.
I was taught to filter BaSO
4 on paper and then to roast solid, which can't be done on sintered glass
Is there iron in your sample? If so, you may try to reduce it to Fe
2+ before precipitation, to lower coprecipitation problems. Adding some EDTA may help to mask coprecipitating cations as well.
Hmm, Alberto posted about aging in the meantime... that happens when you must answer some calls while editing post