Theres a few things you can do.
First of all, take Contagion in your 2nd row talent. Absolute Corruption is only taken on aoe fights.
Don't wait for 2 shards before you do your cycle, if you only have 1 and theres no UA up, spend it and drain.
You should be reaping when you don't have it up for your UA's or its going to drop before UA's finish.
When I was gearing my lock, Siphon Life was better regardless of sets, so maybe play with that.
Outside of all that, it just comes down to RNG with shards and souls. It's more forgiving on fights with adds because you can drain them and get shards if they die while you drain them. This also gives you souls to spend so make sure you snipe dying adds and tag them for souls.
Check out this website for all the info you need.
http://lockonestopshop.com/#!/Affliction/Introduction
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For ST, this is wrong. Also Agony isn't even a main source of damage, its usually equal with Corruption and Siphon (Unless you have the helm) It's all about the UA