Use heavier/sticker grease. A lot of new lubricants out there are being sold as "grease" in bike shops but are sometimes too thin, viscosity-wise, to hold loose bearings in place in cups durng installation. I always used Phil Wood "Waterproof Grease" to repack bearings and I never had problems holding the loose bearings in place in the cups as long as I put enough on it and the bearings.

Chombi