When you consistently use the same dose, even if it is one which would normally elicit sedative/opiod effects, tolerance increases and the effects change. You will have to up your dose to achieve the sedation you desire.
Pretty much sums it up. I've been using kratom for over a year now, and while I was only using it a couple times a week, I barely ever noticed a tolerance hike. 2, maybe 3 times a week would provide STRONG opioid effects, extremely euphoric and noddy, just lovely. Then, I started using every other day, and just as you said, my highs started to become less and less 'narcotic' and more 'uppy' for lack of better words. Now, I use kratom about 5 days out of the week, and my tolerance has skyrocketed. I barely get any narcotic effects from it at all anymore, I simply get a mild boost of motivation, anxiety relief, stimulation and endurance, but that's about it. I definitely should take an extended break and then start back up only using a couple times a week again. You should probably do the same.
One thing you could try is buying some Tagamet (cimetidine, an OTC antacid) and taking two (400mg) about an hour before dosing on kratom to potentiate the effects. I find this boosts the mental/euphoric effects big time, and I never dose on kratom without it. Many will tell you it's placebo, and I personally don't care if it is because it most certainly works for me, placebo or not.
Another thing to try is eating a nice size absorbent meal (like a Subway sandwich or a hamburger), and then dosing on a higher-than-usual dosage of kratom. I find that having your stomach full will help slow down the absorption of the kratom, thus alleviating most of the nausea that accompanies higher dosages and also extending the duration of effects. Of course, this doesn't really work if you use extracts. You'd need to use a plain crushed/powdered leaf for this to work right. Taking an extract on a full stomach might be a complete waste.
I hope it helps! Kratom is an incredible plant if treated with respect. It will also bite you in the ass if you don't.