Glamorgan suffered a nine-wicket defeat at Hove as Sussex cruised to the target of 147 thanks to half-centuries from Phil Salt and Luke Wright.
Glamorgan remain without a win despite David Lloyd recording his second half-century of the tournament so far.
"We have had a few difficult nights in this competition this season and tonight we ran into a good side playing confident cricket - which is not something we are doing at the moment" said captain Colin Ingram.
"Once again I couldn't fault the effort of the guys but we know we can improve.
"Personally, it's been frustrating for me because I have had a good couple of years with the bat in this competition and it's not quite happened so far. But I don't feel I am far away, I'll just have to keep grinding."
"It was the same as it's been all year, we were probably 40 (runs) short on that wicket." added head coach Matt Maynard. "We didn't get any sizeable partnerships with only Lloyd getting in despite a bit of rhythm between (Colin) Ingram and (Kiran) Carlson.
"It's not happening for us but it won't stop us trying to put it right for the rest of the competition.
"They talk about the game being played in the top two inches; it's about trying to remain calm while you're going for the boundaries, and when it's not coming off (for batsmen) you tend to try a bit too hard."