If your hair feels flat and greasy, it could be due to the following reasons:
1. Over-production of sebum
Sometimes, the scalp can be genetically predisposed to produce more sebum than it normally should. This overproduction of sebum makes the hair flat and greasy.
This condition can be cured using natural remedies along with medications.
2. Product build-up
Your hair may feel flat and grease when you have product build-up from shampoos, styling gels, and conditioners.
Avoid hair products that build up on the hair such as dry shampoo and those that are alcohol-based.
Using the wrong conditioners and shampoo for your hair type leads to product build-up on the strands and scalp.
If any product is trapped on the roots of the hair, it leaves the hair feeling heavy, dull, and of course greasy!
3. Heavy products
Heavy products on the hair lead to a higher chance of experiencing product build- up, in turn, making the hair flat and greasy.
Some products contain heavy and harsh ingredients such as waxes. These strip the hair of natural oils making the scalp overproduce oil in turn making the hair greasy.
When using products apply them in moderation or look for light-weight products for your hair.
4. Using the wrong hair products
Hair needs conditioning to keep it moisturized, lively, and healthy. However, if you have oily hair, you do not have to always condition your hair.
Avoid applying conditioner, waxes, creams, and oils to the roots of the hair, as they make hair greasier.
Remember these products are made of emollients and too much of them can weigh down the hair making it look lifeless and greasy.
Since the scalp produces its natural oil, there is no need to add conditioner to the roots of your hair. When washing oily hair, focus the conditioner on the lower half of the strands only to make it more effective.
You may just be the cause for your flat and greasy hair, thanks to constantly touching your hair.
Avoid touching or brushing your hair frequently as this stimulates more sebum production.
When you brush your hair, it increases blood flow to the hair follicle, but it also stimulates sebaceous glands to produce more sebum.
More sebum production leads to greasier hair. Stick to brushing the hair before washing it and when doing that ensure the brush tips reach the scalp to get rid of any product residue or dead skin.
Touching the hair frequently also transfers oil from your hands to the hair, making it greasy.
6. Nutritional deficiency
Another common cause for flat and greasy hair is nutritional deficiencies.
Hair is made of keratin, a protein that is also in fingernails, and around 12 percent of hair is water. 
This means hydration and diet are crucial for healthy hair. Vitamin E, a part of sebum, is essential for the prevention of flat and greasy hair.
Oils such as avocado oil are rich in Vitamin E and go a long way in moisturizing the scalp and preventing excessive oil production from the scalp.
Foods such as leafy green vegetables, sunflower seeds, pumpkin, nuts, meats, and cereals are rich sources of Vitamin E. Vitamin B complex and iron also aid in promoting a healthy scalp.
7. Improper styling
How you style your hair may cause greasy hair.
Tight hairstyles make hair greasier at the roots. There is an accumulation of dirt and oils on the scalp underneath hairstyles such as ponytails.
This will lead to hair being flat and greasier. Avoid using too many styling products as they clog up the scalp. Your hair needs to breathe hence try hairstyles that give it room to flourish.
8. Excessive heat styling
Another cause for flat and greasy hair is excessive heat styling which leads to higher sebum production.
When you blow-dry your strands, the hair becomes oiler because straight hair allows grease and oil to get into the hair shaft.
A hot steamy shower feels heavenly but causes the scalp to be dry from overexposure to hot water, resulting in a dry scalp and overproduction of oil to compensate.
Go easy on heat styling appliances and when washing hair, stick with a medium-warm water temperature.
The final rinse should be with cold water, which closes the hair cuticle adding shine and luster to the hair.
9. Going too long without washing
If you have naturally oily hair, it could be that you are going too long without shampoo.
Going too long without washing contributes to flat and greasy hair.
When washing oily hair, ensure you use a gentle clarifying shampoo once in a while to remove any build-up or residue.
Not washing the hair thoroughly causes a build-up of skin cells, oils, and sweat in the hair.
In a nutshell;
Your hair is flat and greasy because of the following reasons;
- Over-production of sebum
- Product build-up
- Using heavy products
- Using the wrong hair products
- Nutritional deficiencies
- Improper styling
- Excessive heat styling
- Going too long without washing