Hard to access without more information, but I'll give it a shot anyway (talking out of my...)
Beginner-intermediate routesetters finish every boulderproblem with a long move to a jug, so I will assume that the boulders you struggle to finish are around 6-8 moves long with a hard first move and a morpho last move (so the last move may be easy or hard depending on your stature). The most likely cause of consistently failing on the last move is either physiological (you're falling at the 25-30s mark because your anaerobic lactic metabolism is not powerful enough — you can only do the move using a far amount of the most powerful muscle-cells which prefer the ATP-PC system, which will not be available after ~10-15s of climbing) or mental, but the training to correct the problem is the same anyway.
Do the following 1.5-2 x week for 8 weeks: 3–5 X [4-6 X 2-3 move] ; r:10s, R=3min.
(A set/work-unit consist of selecting one boulderproblems of 2-3 moves and repeat it 4-6 times with ten second of rest between each repetition. Rest 3 minutes between each set and do 3-5 sets). The problems should be fairly hard but not maximal, you should not get pumped so stay within 15 moves for each work unit. You should only fail on the last or second to last 2-3 move problem on the last set. You should fall because you power-out (not because you are pumped). Make every rep count.
After this exercise you will not be able to do any high-quality training, so do it after short bouldering if you combine it with something else. After this you can do easy traversing etc.
[Since I haven't seen you climb this might be bull, but hey it's free advice so basically worthless