It's usually a headache to prep old soft concrete floor.
Soft floor is usually due to more than half of cement was faded away by years, sand & aggregates exposed. Due to lack of the enough cement, the poured materials are loosened by time, holes come out after pass of grinding process. Sometimes the aggregates are easily removed by hand.
The better process would be applied more times of desifier in order to re-bond the
aggregates and sand.