Just been looking at 1916 Wilmers and noticed the house I lived in isn't in it, considering they were all terraced houses
I doubt it was a vacant plot of land.
Did the compilers of early street directories go from door to door asking questions as in a census or would they have taken off an all ready created list - possibly off council.
Maybe the missing houses were empty, so no details available so left off.

This is from the Leicester University Historic Directories site:

Origins
In Britain the first recognisable directories emerged during the late seventeenth century, meeting a growing demand for accurate information about trade and industry.

Why was there such a demand? The answer lies in the expansion of commerce during the period. There was an increasing number of tradesmen, many of whom were becoming more specialised and forging business links.

Early publishers
Early directories appear to have followed two paths of development:

1.Some early directories were speculative ventures. These were established by entrepreneurial publishers in response to the expansion of trade.

2.Other directories evolved from the lists of traders kept by the earliest registry offices. This type of directory was particularly common in provincial towns.

Directory publishers during this early period came from all lines of work, which gave them access to information about names and addresses. Some typical examples include:

•registry officers
•printers
•house agents
•auctioneers
•policemen.

The ways in which publishers collected data also varied considerably. Some obtained information by personal canvassing and combined the results with existing listings.

Other publishers simply asked people to send in their names together with a small payment if they wanted to be included in the directory.

Wider geographical coverage
Another important development was the emergence of larger-scale directories during the late eighteenth century. These covered substantial parts of the country, in contrast to previous directories which had focussed upon a single town and its surroundings.

Such ambitious publications were costly to produce, requiring collection of data by a large number of local agents. Consequently the production of these national and provincial directories was increasingly concentrated into the hands of a few large companies.

The Post Office
By the early nineteenth century methods of compilation had become more organised. In part, this reflected the growing links between directories and the Post Office. Many postal officials, such as Frederick Kelly, turned their hand to directory publishing as a means of both aiding their work and making some extra money. Information was collected by letter carriers, who circulated forms during their postal rounds, and also delivered the finished directory on commission.

thanks uptocx - as usual a concise and clear answer
But
what about the missing numbers?

One possibility is that your house might have been some kind of home for unmarried mothers i.e. nursing home.

My grandmother was born illegitimate to a lady of means and the address given on the birth certificate is not shown in the street directory of the time although it was definitely there as it still is.

These would have been for well to do families, the common folk would have had the workhouse I assume?

Where or how can I access Wilmers directory? It sounds right up my street, excuse the pun.

It's a cd that's sold just google Wilmers Street Directory
Mainly Birkenhead though

You tend to get "missing" numbers when the property was vacant.

During the first world war there was an absent voters list for electoral rolls.It could be worth checking. They are in ref section of Birkenhead Library.They have information regarding Name Regiment and number.
Alan

I don't know about later directories, but the very early Gore's Liverpool Directories were certainly compiled by the publisher sending men around the streets to make lists of all the properties and their occupants. Somewhere I have a copy of an entry in the Liverpool Town Books, I think from the 1790s, recording a complaint from Mr Gore that his men were having problems because so many of the properties were unnumberered (the Corporation had some years earlier decreed that all houses in the town should bear a visible number), and asking permission for his men to be allowed to paint numbers on the houses as they went round!