2015 Kia Optima Ex 2.4l AC problems

I have had my 2015 KIA for about four years and the AC has
never worked great in it.  I took it to four ac technicians and
all said the freon was low and the compressor was shot.  
Bought a new compressor and had the technician install it
and worked like a charm for about three months.  We have
cool fall periods in our area so I did not use the ac at all
from Oct to April of this year.....went to turn it on and back
to low charge and hot air.  The mechanic who installed the
compressor did the dye/gas test with Uv light on it and
couldn't locate a leak.  HELP!!!!!