I saw there was a topic said that pxxpers diapers always happen the case as you mention (under the subject - baby's product) few mos before. The mami there said this was because of the "pxxpers" diapers not so good...... Last time I also use "pxxpers" and the urine really "came out" quite many times, I don't know if it is really cause by the diapers or what else, but after I change the brand (huggxxx) it never happen again.
Maybe you try another brand or change a bigger size diaper for your b.