In some cases, an old building may not be able to achieve 50 Pa. of pressure for a proper test. This situation is described by the abbreviation C.R.F. - Cannot Reach Fifty.
There can be many reasons why this is happening:
All openings to the outside environment haven't been closed - do a thorough check of the inside and outside of the building. The older the house, the more odd openings there could be - ash dumps, cistern outlets / inlets, coal chutes, etc..
There are cracks and openings in the building envelope that have developed over the life of the building - plaster & lath, settling, rodents, etc..
Windows and doors are not air tight when closed. If seals have failed or if the unit is an older wood construction, there could be openings in old joinery or around where putty has loosened on glass panes.
Failing or missing mortar work in solid brick or stone construction.