Pokemon diamond Poke Radar/Chaining pokemon

Question

i have a lot of questions for use the poke radar and about chaining

1) why does my chain keep randomly breaking even when its the right pokemon? (ex. when i have a chain of 15 gastly going then the watch thing says the chain broke when i found the same pokemon and my repel hasn't run out)

2) whats the difference between grass that shakes and grass that shakes and sparkles a little? (not shiny patch grass)

3) do certain pokemon only show up in certain kind of grass (ex. say i find a chansey in shaking and sparkly grass, will i only find chansey's in that grass or will i also find it in shaking grass too)

Answer 1

not sure bout 1st one

the  diffrence in grass is not rare pkmn are in the non sparkly in sparkly

non sinnoh dex pkmn or rare s dex pkmn

hope i helped