Humn... might be just the Flasher relay needs to be changed to an LED type or to add resistors to each of the turn signals so that the proper resistance for the stock flasher relay. The problem with most LED flasher relays they are generic and you get to play wire roulette to get them in the correct position.
It may help to know what bike we are discussing; for example, were the front signals also running lights, that is, on but dimmer when not flashing? If so, did they have three wires, or use the frame as ground to light the lights? How many wires do the new signals have, and do they have running lights? How are they grounded?
Not flashing is probably because there is not enough power drawn to make the flasher work, so you would need a LED type flasher, as Aphrodite explains, or just load resistors of high Watts and low Ohms.
OK, I found a manual with the wiring diagram, and it seems the front signals do not have running lights included, just the output of the flasher relay through the L/R selector. So, if connected the right way around, both the front and rear left signals should turn on at the same time when selected, as should the front and rear right signals. No flashing is down to the common flasher relay, but no light up front would be a wiring issue. Because of how the indicator is wired, you may get some light on the right when the left is selected, or some on the left when the right is selected; the indicator uses the off side as ground, and LEDs don't do that well.
The lights are running but the front lights are very weak but rear are bright and they don't flash they just lights on. They are grounded with 2 wires from the frame and the turn signals have 2 wires too
According to that diagram, each signal gets a wire from the switch, and a ground; it doesn't show if those wires are doubled up. The front signals are not getting full power; why depends on the wiring.
If you are anywhere near St. Louis, MO, I could help hands-on; using text, at this point, is very hard.
I suspected you weren't in the US, much less near me. You are going to have to go through your wiring updates, and verify everything against the wiring diagram. After you replace the flasher relay with a LED-capable type, of course.