While fireflies and firefly queens and such are too strong to have this matter, just in case the opponent was also spamming rustlers or dragonflies or whatever, keep in mind that creatures with power lower than or equal to the damage reduction of the shield they're attacking won't actually attack and therefore won't ever be frozen. So, if their field was full of creatures with 1 power, that would be distorting what you were seeing in terms of overall freeze rate.
Also, if they had shards of freedom, those can allow flying creatures to completely bypass shields, which would also result in few to no actual freezes.
That said, though, still just bad luck on all the >1 power creatures (assuming no shards) like higu said.