Dissolve Polygons by attribute leaves empty spaces

Question

I am trying to dissolve polygons based on the same attribute using QGIS 2.8.2 (Wien). I use Vector, Geo Processing Tools, Dissolve and then I select the attribute. The data is Postal Sector in Uk and I need to create Postal Districts, in the attribute table I have created a field called District and I use this field to dissolve my polygons. The problem that I have is that the new District layer has some empty spaces. I have taken a screenshot of one of them:

The Sectors CV21 1 and CV21 2 are not part fo the new District layer and if I click on them using the "selection" icon, nothing happens. My guess is that QGIS cannot dissolve them because CV21 2 has an intersection (exactly where its label is). See screenshot of that area, note that those 2 lines are crossing but there is not a node:

Can anyone: 1. explain why Dissolve does not merge those polygons together 2. suggest any solution (maybe splitting CV2 1 into 2 smaller polygons and then merge them afterwards?)

Answer 1

I have used GRASS v.clean with method bpol to clean the postcode sector layer. After that I used the Dissolve method as explained above and those "empty" spaces disappeared. As far as I have checked, now my district level is correct.