What type /brand of sealer did you use?The problem is if you've used a rather thick sealant such as HG IMPREGNATOR or HG MARBLE IMPREGNATOR,using another product such as HG EXTRA (which is basically an 'acid wash'),it may remove the seal-all depends how porous the tile is to begin with.
Do you know which country the tile was manufactured in?
There's every chance it was manufactured in China or Brazil,two countries which are known for using far cheaper raw materials than Spanish or Italian countries.
It could have been that the tile needed more than one coat of sealant.
This problem is well known with POLISHED PORCELAIN as well.
Basically the more 'microscopic' the powder used when manufacturing a porcelain tile,the less porous the tile will be(cheaper raw materials mean 'chunkier particles' which when compacted and pressed during manufacture,will mean more porosity).
Lithofin is a good sealant,but try and track down a stockist of SOVEREIGN STONE SEALER :It smells like 'peardrops' and is excellent for polished porcelain-the excess liquid begins to evapourate,and once buffed with a dry cloth,brings back the polish!
Sorry,rambling on here,try HG REMOVER with your grout problem mate,if that don't work,I'm afraid it's something stronger......
