Biofilms are certainly the main reason why probiotics are ineffective IMO.
I suggest that you try a good probiotic with the soil based organisms such as Bacillus Subtilis in it because these have bacteria which not only produce anti bacterial compounds but also produce enzymes which can break up the biofilms and therefore have a two way attack on the bacteria. Removing their protective biofilm and then attacking them with anti bacteria commpounds. Once done, when they have been destroyed, other bacteria can take up their place and implant.
There is not much room for new bacteria in the intestine. Think of it like a housing estate where most of the homes are owned and therefore people cannot move in. If someone doesn't keep up repayments, building a biofilm, the bank ejects them, Bactericides, then new people can take up residence, probiotics.