It would be nice to think its just the weight pulling it down but its an average size, little smaller than 6 inches. and its a consistent thing, theres never a time where it will stand up more than other times.
The amount of skin covering your penis has nothing to do with it. Erections are caused by chambers inside the penis containing a sponge like tissue that fills with blood and becomes firm.
I'm no expert, but there are probably a few possibilities. Are you taking any medications? When this happens, are you not as horny as usual, or not getting as much physical stimulation? The mood you're in can also affect arousal - it helps if you're more relaxed. Or has your penis gotten too heavy? :)
To give a better describtion, its as if my erection is at 90%. Firm but just falls over when laying down or just hangs down when standing