Cycling is a quad dominant sport and many cyclists can become quad dominant in their everyday movement patterns which can affect body posture and core stability muscles such as glutes. If your main daily activities are riding your bike and sitting down in a slouched position you may find that over time you may lose the ability to effectively fire your gluts.
In addition to this, Glute recruitment is also affected by hip position on the bike, which in turn can be affected by the saddle angle, saddle profile, flexibility of the rider along with individual movement pattern habits. Many modern saddles have a flat profile, which can encourage you to roll the top of your hips backwards, which can inhibit core muscles and turn off glutes from firing during the pedal stroke. In contrast, saddles with a swooping profile allows the top on the hips to be rolled forward which encourages glute recruitment. If you have never ridden this type of profile it can take some time to get used to, as you may instinctively want to roll the top of your hips backwards out of habit, which result in contact hot spots and discomfort.
You said you were able to get your glutes to fire at faster speeds (i.e., 30+ kph). I assume this is seated harder efforts. Under harder efforts you are concentrating more, sending a stronger signal down your central nervous system which may be enough to recruit your glutes. Lighter efforts the signal may not be strong enough. This could indicate that your glutes have become weak, and/or your general ability to fire glutes has declined. The only way to really reverse this is by practicing daily exercises designed to help you fire your glutes. Also when you are climbing out of the saddle focus on rolling the top of your hips forward when standing this will help encourage your glutes to fire .
Not sure what is going on with your calves. It may reflect cleat placement (I.e., foot position on the pedal). It may be unrelated to your glute firing issue. Another possibility is that it comes out of compensatory behaviors that result from poor hip position and strength.