I do not think you are doing anything wrong.
Especially since you have already been successful at breeding related fish.
They may be a little young or maybe you are being a little impatient. Right now I suspect it is a little of both.
50% water changes once a week is a good routine amount of water to change.
I usually change about 3/4 of the water every week then increase that to 75% every 4th day if I am trying to encourage spawning. I am not trying to simulate any seasons but rather I am trying to provide the highest water quality I can.
I also increase the amount of live or fresh frozen foods when trying to encourage spawning. Your catfish love small or chopped live worms and Chironomid larvae(AKA "Blood Worms").
All these are just small things one can provide which may help but ultimately being persistent and patient are the real "secrets" to breeding most species of Loricariidae. Too much is made of trying to simulate the seasons.
Their habitat is much cleaner and oxygen rich than the best aquarium. The Atabapo is one of the haunts of the Pterophyllum altum Angelfish which is among the most demanding of aquarium fish and Altum are never found in less than pristine water. Your Loricaria species share the same habitat with P. altum but are far more adaptable.
Your fish may not be too demanding about their water chemistry but the habitats in the Atabapo are warm; 28/29*C and their water type is black water which has almost no measurable E/C and the pH is normally between 3.5 and 4.5. So at least continue using your soft water and perhaps tint it by using a few T. cattapa leaves as needed. You need not try to exactly duplicate the water chemistry but I would recommend that you continue using your moderately soft water. I recommend keeping them over a fine sand substrate 2 to 3 cm deep. Be sure to leave a lot of open sandy area along with some overhead structure like a few branches of wood; not quite a cave but some form of sheltering structures. They also need high dissolved O2 levels so provide plenty of aeration and noticeable currents. They will like partial shade from floating plants like Tropical Hornwort, Water Sprite or whatever grows well for you.
I highly recommend that you read this excellent article written by Rupert Collins about his experiences keeping and breeding the related Pseudohemiodon sp Marbled as a good baseline reference.
http://www.planetcatfish.com/cotm/cotm. ... cle_id=361
Your patience will eventually be rewarded. Best of luck to you.
Avid Trout fly fisherman. ·´¯`·...¸><)))º>