just means they have a low probability of producing an egg, but you will get one eventually. The rate at which you get eggs is based on 2 things: ID/OT and the actual pokemon.
Different ID/OT same pokemon (charmander and charmander):- High
Different ID/OT different pokemon (charmander and dratini):- Medium
Same ID/OT same pokemon:- Medium
Same ID/OT different pokemon (*ditto and poliwrath):- low
*this is likely your scenario
I fix this with this:- as soon as an egg hatches with the opposite sex to the pokemon you're breeding, swap it with ditto i.e. if your poliwrath is male, as soon as you hatch a female poliwag, swap it (be aware some pokemon can't breed using this method: single gender only like tauros or non breeding offspring like cleffa)