As mentioned above, several factors at play here. Whenever I change a major process I get lower efficiency for the first few batches. After some experience the efficiency rises back to an acceptable level.
Here are some ideas:
Efficiency: My MLT usage instructions says it is designed for .67 gal per min but says efficiency can be increased by slowing down the flow. As mentioned, another mfr recommends raking the top third of the grain bed every 15-20 minutes. Sometimes I rake, most of the time I don’t. I haven’t really noticed a difference but it’s supposed to prevent channeling.
Stuck recirculation: I use .5-.6 gpm to recirculate during the mash as monitored by a rotameter in the flow path. As flow slows, I reset the .6 gpm, it slows again, I reset again. After a few iterations of slow/reset, the grain bed compaction and differential pressure above and below the false bottom stabilizes. The flow is steady thru the remainder of the mash. A grain bag helps with flow and a manometer can be used to monitor differential pressure. I use a bag but not a manometer.
Mash thickness: I like a thin-ish mash. I routinely use 2.x qt/lb liquor/grist ratio. I believe it assists in recirculation.
Crush: Milling your own grains at home will allow you to dial in the perfect crush for both your brewing system and the grain you’re using. When your grain is milled properly, you’ll move closer to achieving consistency and maximum brewing efficiency.
Grain is an agricultural product. It’s size and friability can vary each year depending on the yearly harvest and the maltster. Likewise, mill gap settings are not universal, and while there is not just one “ideal gap setting,” if you’re using a 2-roller homebrew mill, you can start with a gap setting of a credit card and adjust from there.
I use a 60/40 crush (60% remaining on top while 40% falls thru a No14 sieve). But it took several iterations to get the balance I like between flow and efficiency. I started at 70/30, moved to 65/35, and landed on 60/40. I could probably move to 55/45 but I’m ok. I doubt it’s a $2 difference in grain cost vs. a stuck recirculation potential I’m unwilling to risk. You’ll find your mill gap after a few brews.
RIMS: I used to use a RIMS. I found there is a balance between flowing fast enough over the element so you don’t scorch the wort and a stuck sparge from flowing too fast. Even though I brewed good beer with the RIMS, I switched to HERMS to eliminate this additional balancing act coupled with the fact that the wort doesn’t come in contact with the element.