"If you are grateful, I will give you more" Al Quran
Things that usually block gratitude:
1) You subconsciously think that creation and everything in your life is happening randomly
If you associate everything to random events (no Creator's role ), you cannot give credit to Source whom everything relies upon. Creation is the domain of the Creator - Allah. Even if you are not a religious person, you will have to admit that nothing in the universe is happening randomly. Even a simple act of creation (such as walking and sleeping) requires infinite knowledge of science and non-physical laws that are beyond the comprehension of human mind.
The whole idea of gratitude is that you have to give credit to an outside factor (not you) that did you a favour. This article you are reading right now is a good example. The eyes that let you read the words are having an unlimited number of processes going on (from retina to image reflection and then transmitting that image to your brain etc), the gravity of Earth that is making it possible for you to you sit still on your chair and use the computer/mobile phone to read this gratitude article are acts of creation that are totally beyond you and you have no control over anything happening inside and outside of you. So why not be grateful to the Source (Allah/God/Creator) that operates everything with precision and great care?
2) You think you are "entitled" to get what you want
There are millions of people in the world who are in much worse conditions than you are in right now. People lose life accidentally - people get attacked by chronic illness and life-threatening diseases - there are people who don't even have shelter/home (in Indian capital New Delhi, about 200,000 people sleep on streets - a news report). Entitlement is one of the biggest blocks that keep us from giving thanks to the Lord and Creator of everything (all events and functions in physical and spiritual plains). Things can get worse without a reason.
2) You take things for granted
3) You love your ego
4) You are always wanting to be perfect
5) You are impatient and you want quick results
6) You complain too much
7) You overestimate you abilities (underestimating isn't good, but overestimating your power and ability creates arrogance and self-obsession that ultimately lead to ingratitude)