Low signal quality strength on some sky channels

I have recently installed a sky mini dish using a signal finder. I thought all was well after checking the signal quality on the receiver which was around 75%.

However I noticed that some of the channels were freezing slightly during bad weather. Checking the individual problem channels the signal quality is very low about 20%. I thought that the initial findings of 75% would be more or less the same for all channels.

So how do I go about getting a better signal strength for these channels?

An old fashioned way to fine tune signals from a "strong" satellite eg 28e, is to place a wet facecloth over the lnb after youve found the satellite with the finder and tweek. Only works on strong signals mind.