qgis - Clip polygon using buffer

I'm very new to QGIS, so apologies if this is a daft question. I've been trying to resolve my problem for a couple of days and I can't get anywhere. I have searched for similar issues here on SE but no-one seems to have the same problem.

I have a layer of forest habitat patches (green patches) and a boundary line layer (in purple). I want to determine which forest habitat patches are within certain buffers from that boundary line. I have used "Vector > Geoprocessing Tools > Clip" in order to create a new .shp with forest habitat patches within a buffer. I am getting inconsistent results, it seems to work for some distances...

But in others it misses patches (highlighted by the red square)...

Can anyone tell me why this might be happening? And any way to resolve it?

I have tried to construct workarounds using "Difference" and "Intersect" but I get other inconsistencies there as well.

I'm using QGIS 2.12.0, I have checked that all my shapefiles are in the same CRS, and checked that there are no Geometry errors.

Answer

Right...it seems that there might be an issue with QGIS 2.12.0 (and QGIS 2.10.1) not processing your shapefiles correctly. I ran the clip tool in QGIS 2.8.2 and it seems to have worked (atleast for the area you have highlighted in your question):

• The clipped layer using QGIS 2.10.1 and 2.12.0 (shown in red):

• The clipped layer using QGIS 2.8.2 (shown in green):

It is rare for some tools in older QGIS versions to do better than the latest ones but for this instance, you could try downloading QGIS 2.8.2 and doing the same thing.