GX7 Problems with off-camera flash and trigger

Hi, everyone

I just started to try off-camera flash photography. I invested in a Yongnuo YN-560 IV, using 560-TX for Canon as trigger.

The problem is, the off camera flash won't consistently fire, sometimes it will goes off, sometimes it won't. Plus, the multi mode won't working. At first, I think the flash is the cause (since I bought it on a bargain price with only 7 days warranty) but after I tried it with my old 60D, I realized that the cause may come from my GX7.

When using it with Canon, just half-press the shutter will send the radio signal to the flash to 'ready'. The flash will always fire if I fully press the shutter.

During multi mode. 1s exposure.

This is when I tried to use multi mode flash. The flash fired 8 times, just like I expected.

This is the couple with my GX7. Nothing happened when I half pressed the shutter. When I fully pressed it. Sometimes the flash LED changes to red and the flash fires. Sometimes it just become 'ready' (LED being blue just like in the first picture).

I think the problem is how the GX7 shutter 'think'. They'll just send a pulse of signal to the hotshoe the moment I fully press the shutter. Sometimes it's fast enough for the flash to fire. Sometimes it isn't. Plus, it'll just fire once in multi mode.

My question is as follows

1. Is there a settings that maybe I'm missing? (I know I'm using mode P in the picture but normally I'll use mode M since YN560 is the manual flash.)

2. Is this happened with other users with GX7? (So if it's just my defective hotshoe, I'll have it sent to the service center. This boy is still in the warranty period.)

3. If this is normal for all of GX7s, which equipment do I have to change in order to make it reliable like when it's working with Canon? (even if it's new body, please tell me your opinion)

Thanks in advance

James