Help with very slow preform

Hi!

I´ve been preforming a kinda big (currently 15mm) amethyst but the girdle facets refuse to move. I´m using 240g diamond (my roughest lap) and the pavillion facets preformed just fine, but the girdle facets just reached a point where they won´t go any deeper (I´ve spent over an hour on a single facet and it didn´t even move a millimeter). There´s four girdle facets and I still need to remove quite a bit of material to make room from the crown.

I´ve already tried varying rpm and waterflow, but that didn´t work.

Any advice? Thanks in advance!