We are travelling from Venice to Lake bled in July. Catching a train (railjet) from Venice to Villach (Austria) and then another train (OBB) from Villach to Lesce-Bled (Slovenia.) The latter train makes a stop in Ljubljana. Weve booked our tickets online through OBB website just to be on the safe side. There are buses along this entire route too with a change in Villach.
You can also try goopti which provides private and shared taxi service from Venice to Ljubljana.
From what Ive read the plans for a train from Trieste to Ljubljana has been delayed till September.
Hope you have a nice holiday!
Although I've never used it I believe GoOpti provide a bus service at around 3hrs duration Venice to Ljubljana. Another option is train to Trieste and then bus (about 1.5hrs) from there to Ljubljana. The most scenic option is Venice-Gorizia (train)-Nova Gorica (bus)-Ljubljana via Most na Soca (train) but it takes time.
We travelled from Venice to Ljubljana by GoOpti taxi/shuttle service on 25 June and back on 2 July. Book in advance. They will contact you by sms/text message 24 hours before your arrival and give you the exact pick up time. Go to their website and make your choice, as there are different obtions at different price ranges. We were very happy with their services. Just be aware that some drivers might be at the pick up point 15 to 30 minutes earlier than arranged. In our case the explanation was that it was to allow for traffic jams on the highways. (Fortunately we are always rather early than late!)I will definitely use then again.
I forgot to mention: we met a fellow traveller at the Bled bus station who was travelling between a number of European cities by Flexibus. So that' s another option to explore.