[SOLVED]hdmi show screen only if plugged before the boot

[acconciox]

the problem is gone, probably due to my mistake or misuse.

hi im running raspi pi 3 B, i cannot remove/change screen on the run, when im not on the X graphical mode, after startx the hdmi plugin is detected.
the HDMI to VGA converter (with hdmi sound) is not detected when i plug in (on the hdmi port of rasp pi 3 b) maybe i have to install some firmware or package? before arch armv7, i started with raspbian and this possibility was useful, but arch seems faster on other tasks.