You will need to write down and reply on this forum the instructions that should be on the emulsion tub, before i can come to any conclusion.
errrrr..... there aren't any instructions on the tub in relation to thinning it down !
OK so we have established it REALLY cheap stuff then!
The mist coat (primer) will look patchy..in the sheen for two reasons..
One..you have applied it unevenly...the light bounces of different surfaces in different ways..just look at plain carpet tiles..they look like the have a chequered effect even though they are the same...thats because they have been placed against each other in different ways light is bouncing off them differently...
If you have a wall that has some areas that are quite textured due to not laying off properly with a roller and thers that are flat..then youl have a similar effect..
Same if you do them with a brush and overlap each section...the overlaps always look lighter because they are smoother..and have more paint on them..ever looked at the side of someones house after they have painted it with a brush and seen loads of squares???...now you know why..
Give the walls a good rub down with some 100 grade scratch and then put two coats of your decent paint over the top...one isnt enough as it will sink into the mist coat unevenly and give you more patches...the second coat will seal the surface...the thrid will finish it nice.