Forgive me if this is too subjective, however, I've been doing some mtb riding and noticed that it's always my glutes that fatigue first especially on climbs, talked over with my friends and they tell me it's the last thing to go.
I'm wondering am I spinning too low a gear or perhaps pushing instead of making circles. How can one ensure they are engaging all muscles through the pedalstroke?
Striving for an objective answer, I would avoid venturing into the "get fitter hurr durr" types of answer.
Muscle engagement on the bike is very much dictated by various parameters including seat height, saddle fore/aft, crank length etc. You would find that lowering/heightening your saddle by an exaggerated amount engages different muscles and consequently causes pain.
Now I have some time, I'll attempt to highlight some stuff. But long story short, there is no definitive answer and there will be some subjective stuff here.
Here is stuff that affects muscle engagement:
Physiological issues - Muscle activation is the first thing I would look at in this area. Perhaps those muscles are overcompensating for others purely because the others aren't being engaged properly.
Strength and mobility - When we move from one type of excercise to another, often we find that the mucles we build for one, are not sufficient for another. Muscle imbalance is a good one to point out here, notice that imbalance doesn't have to mean "left leg is weaker than right leg", it would mean that one muscle is letting down other muscles which depend on it. Mobility might be a good one to think about too, since the riding position might be different than what your used to, so perhaps your body is not used to this new position
Nutrition - Maybe this is not so relevant for the glutes issue, but it should be considered when talking about "all muscles are engages". I won't go into details here, but if your nutritional intake is not good, then the muscles won't fire in all cylinders.
Bike fit - This is a big topic and bike fit depends on many factors, including your body, the bike and the type of riding. Bad bike fit will likely cause your body to not use the muscles in the way it should. Things which really affect the way your body moves are pedal/shoes, seat height and seat angle, these would be my first things to check, but a full on bike fit would be best.
Cycling technique - (Nothing implied here) Cycling, similar to bike fit, depends on the person, bike and type of riding. Bad cycling tekkers could lead to some muscles being overworked whilst underworking others.
Final thought, firstly just ride more mtb and try to make small changes and note if any of them make cycling any better. Secondly, try and see profesionals for advice, we can't really help much without actally looking at you and how you ride. Finally, there is no 1 stop cure of this, and it might not even be one area from the above points and could be a combination of some and even other things not on there.
There's more I have written about such strength training for specific riding, stretches and breathing, post ride nutrition, hydration and muscles recovery, and there is plenty more. But honestly, there is too much to give a definitive answer.
Good luck!
