It's entirely polarity, if you're talking about #44/#47 bayonet type ones.
In my Bally Vector (modded with my soon to be released sub-board(s) to use LED's on a SS Bally without them randomly turning on and off), it used #555 wedges almost everywhere, and I needed to rotate them to get the right polarity in the controlled circuits (since that was GS), but GI worked either way (since that was AC).
Unfortunately, for the 32 lamps around the sides of the backbox (for infinity lighting), they were all bayonets, and none of the Ablaze 1 LEDs light up.
The answer is above: The basic Ablaze are simply passed through a single diode and as such only work in a DC circuit that is already wired with the correct polarity. The reason they still work in AC circuits is because the diode chops half of the AC wave and converts it to DC and then lights regardless of orientation (but at the unfortunate side effect of causing more pulsing based upon the frequency of the AC power and why cheap Ablazes used in GI have a distinct consistent flicker as compared to other LEDs. The wedge version, since rotatable, will allow you to correctly orient it so that it will light; but does nothing to prevent the flicker under AC power.
The only way to prevent polarity issues is to get LEDs that use full rectification (a set of 4 diodes inside). These always send DC power regardless of how it's plugged into a DC or AC circuit, AND have the advantage of using the full up and down portions of the AC wave to power the LED, thus reducing the flicker seen in GI circuits.
The Ablaze LED's have their place--if you are a LED hater of extreme brightness for inserts, they work great. For GI on the PF they will pulse and annoy you; backbox is acceptable. For general PF GI, frosted 2 mini SMD's from CT or single (3 point) SMDs from PBB are the best (in warm white). No flicker, correct color temp. But these can be overpowering for inserts, and at 28c vs 59/69 cents (appx) in bulk, over twice as expensive.
In both of my games, I have a mix of Ablaze 1 LED and CT (or PBB) frosted for a balanced approach.
In agreement with Phetish -- The Ablaze wedges need their wires bent outward to catch the socket connectors. I had to do the same thing with CT and PBB ones; not really any difference.
For bayonets, actually, I have had fewer problems with Ablaze than with CT. Many many CT frosted ones had their assemblies separate when inserting or removing just 1 time, forcing me to rebend wires and superglue tops on -- again with bayonets! Only a couple Ablaze have I had to glue, and never had to rebend wires inside.