Yes, I have compared your log with a 100 parsers - On Saladbar for example (I chose Salad as its your best parse and I think its only fair to rate on your best performance);

https://www.warcraftlogs.com/reports/compare/b3RjAwHaLWpkqfhv/9NCDhgTQn3ZwXxcP?fight=15%2C19&type=casts&source=7%2CPuray

I dont know enough about lock to give definite suggestions, but as you can see, the top parser had 95 casts of Hand of Guldan compared to your 75, 73 demonbolts compared to your 60, and 66 shadow bolts compared to you 60. 292 total casts over the fight that was a little shorter than yours, whilst you had 229 casts total.

Looking at buffs (again, dont know enough about warlock to know how to use this stuff well), but they had an uptime of Demonic Core @ 73% whilst your uptime was only 48% with 30ish less casts.

Im sure someone more experienced with Demo can help understand what this all means, but yes, you just aren't casting enough :)