I simulated a delta wing and a straight wing in xflr5 with the same profile, wing area and airspeed (very low airspeed. around 10 m/s. it's for a model aircraft). The straight wing was operating at nearly the lift coefficient of the 2d airfoil at a particular angle of attack, but the delta wing was operating at a lift coefficient far below the 2d airfoil coefficient. Why is this so? The wing has a medium sweep of 25 degrees. Will the lift improve with a higher sweep angle?
No, a higher sweep angle will reduce lift even more.
All aerodynamic forces scale with the dynamic pressure, the product of air density and the square of airspeed, divided by half. If a wing doesn't produce lift, move to denser air or speed up.
However, drag will also scale with dynamic pressure, and there is more which influences lift. One is the angle of attack and the second is the suitability of the chosen wing geometry to produce lift at a given speed and angle of attack. Straight wings score best in this respect, at least at subsonic speed.
The sweep angle of the quarter-chord line gives you a first approximation of lift loss. Lift is proportional to the cosine of that angle. Just drive sweep to its extreme of 90°, and the inability of such a wing to create lift should become clear.
Lift is produced by the downward deflection of the air streaming around a wing. When the wing meets this flow straight on, it will have the biggest effect on the air and create the most lift. Any sweep, regardless of forward or backward, will reduce the wing's effect.
The next effect is the influence of the wingtips. If your wing extends sideways a lot more than it extends in flow direction, the influence of the tips is relatively lower. The ratio between wing span and chord is called the aspect ratio, and it figures prominently in the formula for the lift curve slope.
I know this was answered a while back but let me give you my two cents.
As explained by Peter a combination of the wing swept and low aspect ratio affect the lift production of the wing. But another effect also takes place; vortex lift. aeroalias explains it very nicely on this post: What is vortex lift?
In short vortex lift is lift generated by the vortex sheets that accelerate air downwards. And as you said the lift generated by a delta wing is lower at the same angle of attack, but a delta wing is able to go to much higher angles of attack than traditional aircraft until the vortex 'bursts' and the delta wing stalls.
This provides great performance on delta wings when traveling supersonic, but when landing they must land at a higher angle of attack. This is the reason for which the Condorde had a drooping nose. Since it was landing at such a high angle of attack, the engineers had to add it to meet with regulations regarding the ability of the pilot to see the runway.