Plane help

[rcdangar3]

so I have solved my power issue (thanks to you who helped me with that). The next issue that arises is when I launch the plane by hand.
The plane takes flight and then slowly sinks towards the ground. My initial thought is either the cg is too far forwards or there’s not enough airspeed and I’m stalling (I’ve been flying with a slight breeze of 2mph). Can anyone help me?

 

[bisco]

if it is flying level and can't maintain altitude with full up elevator, i'd be looking at battery, motor, prop.

 

[quorneng]

If it slowly sinks towards the ground without a change in nose or nose down then there is nothing wrong with the CG.
If it was stalling it would pitch up a bit, then dive and possibly also roll towards the ground.
If it was nose heavy it would increasingly dive towards the ground.
As bisco says most likely it is a lack of thrust compared to its weight.
How much does it weigh?
What battery? Capacity and number of cells.
Is the battery fully charged with all cells at 4.2V?
I know its a bit obvious but is the prop on the right way round?
Does it feel like the plane's thrust is anything close to its weight when at full power?

 

[Merv]

...The plane takes flight and then slowly sinks towards the ground....

+1 on lack of power.
Make sure you have the prop on correctly, the numbers on the prop facing the direction of travel.
If the prop is backwards, you will only get about 40% of the expected thrust.