During a migration from a FlexGrid to a DataGridView, we encountered a situation where the HorizontalScrollBar did not show.
I found many suggestions like setting a MinimumColWidth value for all columns, etc.
But it wasn’t until my friend Jesus added a line like:
mygrid.DockStyle = DockStyle.Fill
that the HorizontalScrollBar appear.
It might just be that the grid was too big for form but just for the record this is a possible solution.