AFAIK the tp usage was set up like this or that's what DW was talking about back then :
100%-50% : TP used at 300.0 TP only
50-25% : TP used starting at 200.0 TP, chance increases with every AI cycle
25-0% : TP used at 100.0 TP
My memory tells me that 2nd tier didn't exist when I played and wiki seems to confirm it :
"Monsters will use their TP semi-randomly when at 25% HP or above, until reaching 3000 TP at which point they will use it on one of their special attacks. Once monsters fall below 25% HP, they will no longer save their TP and will attempt to use their special attacks immediately upon reaching 1000 TP. Most monsters follow this behavior, but some can circumvent it, usually Notorious Monsters. Note that monsters can gain TP very quickly when attacked by a party (see below)."
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http://ffxiclopedia.wikia.com/wiki/Tactical_PointsMelee tp gains are fine, and usage as described above is what I have been experiencing since the fix last year.
This said, ranged attacks might have another problem which the tp they give? I haven't tested any of that.