Reflow soldering issues
If you must use solder instead of solder paste then you really need to ensure that there is plenty of flux on and around the pads when you reflow. Without the flux the solder won't flow into the components properly.

Solder contains flux in the core, but that burns off as you melt it onto the pad leaving just (almost) pure solder behind. You need flux for the solder to flow onto the metal of the component.

Solder paste is made from millions of little tiny beads of solder suspended in flux. If you're going to reflow you really should be using paste. It's not expensive (well, it is, but you use such small amounts a little goes a long way). Personally I use a nice low-temperature one (138°C melting point) with my home made "toaster oven" reflow oven (ChipQuik SMDLTLFP) in a syringe.

Here's an LED I reflowed earlier today using my homemade oven:



BTW, the pad layout for the LED above is exactly as specified in the datasheet for the LED. Check with the datasheet for your LEDs that the pads you are using are suitable. If they are spaced wrongly (and they look like they may be) then reflowing may be problematical at best.

It's even good for fine-pitched QFN:



I wouldn't even think about doing those with normal solder without having the board swimming in flux when I reflowed.