personally, IMO, the kiddies you mention as being unmotivated dreamers/having memory problems/'thick' may just not be academic learners.
my dh is one, as is I fear my DS2......DH was put in the remedial class, but going on how he is now, that was so the wrong place for him. he is highly motivated, highly inteligent, but in a 'hands on' sense rather than on paper sense.
he has all the same symptoms too as DD2 who is dyslexic, so he may be one of those undiagnosed ones who were missed as children simply because his teachers either couldn't be bothered, or more likely, saw that he daydreamed too much to be helped in the only way they then knew (bear in mind he was at school in the 70's/80's when teaching methods were vastly different!!)
as it stands tho...some kiddies day dream because they learn differently, NOT because they are 'thick'..........IMO!