Having problems coming out of the blocks. Tall runner (185cm). top speed pretty good. 1st 20m sucks! Any tips, cues of excercises (plyos or gym) you can recommend?
where to start… literally 
First I guess we would need to know training age, 30m time, squat strength, pb and current level, age…
In general however:
med ball accelerations, squats, power cleans and frog jumps are all good for that first 10-20m.
If the strength is there then its a matter of working on technique:
hips high in the blocks on set will generally help
focusing on moving the front hand up and out quickly as the first movement, if your starting slow you very well could be thinking of too many things (paralysis through analysis).
keep your head in line with your body at all times
work to come up gradually staying low and making a smooth transition from the blocks to top speed.
Forget where you feet are, legs are, first step lands etc…work on the above and usually a good start will happen when you get those things right.
That is of course if the strength is in place 
Hope this helps!
We need more information.
I have the same problem. I’m 6’2", 196 lbs and don’t come out of the blocks well. What kind of power is needed for a good start? I’m a little heavy right now. I can get down to about 185 lbs, but no lighter. Would this weight reduction be an increadible difference at the start?
My max competition depth squat is 375 lbs.
What are med ball accelerations?
Throw a medicine ball as far and hard as you can and then sprint.
Just to expand, its like a small dip and then push press outward while falling at as an extreme an angle (try to say that 10 times fast) as possible. Then after you’ve thrown your foot catches you and you chase after the ball.