I do have have downloads that is downloading at normal speed, but one is very slow to download at 1.2kb

you are connected to slow peers.

you cant make people give you the file faster than they can.

All you can do if it's not a complete peer, try "unchoke (forced)"
My record is 250bps thats bytes / sec. That's a minute a block, as seen by Bandwidth scaled all the way out. line barely above 0. For hours.
250 byte/sec == 16 KiB / min = 16 min per piece (256 KiB) -- 159 piece left == 40 hours left

You should try looking for another INFOHASH for your transfer. And announce via DHT.