I am sorry for the late reply, but I am glad you figured it out.
Usually the drill files fail to load because there is a decimal point mismatch between the Gerber file and the drill file.
A Gerber file may be formatted as 2:4 - meaning two integers and 4 decimal, so the value 123456 could be interpreted as 12.3456 mm. In contrast, a drill file may be formatted as 3:3 so 123456 could be interpreted as 123.456 mm.
This mismatch means the drill holes will be huge or tiny when loaded in the Voltera software. Unfortunately the excellon drill format is very poorly defined, most drill files do not explicitly state the format being used so it is hard for our parser to correctly identify it.
When exporting Gerber and Drill files it is important to ensure the formatting, and the leading / trailing zeroes settings are consistent between them.
Re-saving both the Gerber and drill file in Gerbv ensures both files are formatted the same.