Dimmers work by varying the trigger angle of the mains.
Figure 1. Phase or trigger angle control of AC mains. Source: Dimmers for LEDs.
How the bulb responds to this depends on its internal circuitry. The incandescent lamps were simply a hot resistance wire and they worked fine. LED mains lamps have a wide variety of circuit configurations - some of which work well with dimmers and some that don't.
Figure 2. Some of the cheap LED mains lamps use a capacative voltage dropper to reduce the voltage and limit the current to the lamps. These generally work OK on a standard dimmer.
Figure 3. A slightly more complex version has a smoothing capacitor, C2, added.
If a smooting capacitor such as C2 is included after the rectifier then the dimmer won’t appear to do anything until the control is adjusted below 50%. This is because the capacitor charges up fully at peak mains even if it hasn’t been on up to 90° into the half-cycle. (See Figure 1.) From 90° to 180° the peak voltage drops so the lamp will dim in proportion.
Any idea what the issue is?
It is most likely a minor difference in components due to production spread in the various components of the lamps. Unfortunately the subject of dimming has got a lot more complex since the conversion from incandescent to LED lamps. Unless you can source lamps designed to work with a specific dimmer the results will be a bit hit and miss.
I have written a little more on the topic in the article linked in Figure 1.