Part of the problem is that Welsh just isn't a priority:

The number of first language speakers is decreasing, often due to having to move away from the more rural Welsh heartlands to find work. Unless they go to a specifically Welsh-speaking job, this means moving away all their Welsh-speaking family, friends and neighbours. They may or may not pass the language on to any children they then might have.

There are lots of child learners i.e. those from non-Welsh speaking families who Welsh medium schools. The situation outside of the Welsh heartlands, in general, seems to be that these kids see Welsh as the language of teachers, homework and exams, whereas English is the language of friends, family and just anything cool. Some argue that although these children have 16 - 18 years of education in Welsh, they still aren't fluent. Yes, they might understand everything said to them, but they're not able to express all their emotions and experiences without resorting to some English. Most (I think) of these children don't go on to get jobs and raise families in the language and many have lack of confidence in their skills.

Adult learners make little impact on the overall numbers of fluent Welsh speakers. It takes a long time to learn a language, and people are busy with studies, jobs and family. Even if they start out well attending evening classes, these pressures of life often take over and fluency is never reached. Those that have the time to give to more intensive daytime classes are often retired and so aren't those that are going to go on and get a Welsh speaking job or start a Welsh speaking family once they're fluent.

Some of these things are generalisations and anecdotal, but if Welsh was more of a priority, they'd be less of a problem. I don't pretend to have all the answers, but I'm just thinking, if there were job opportunties and affordable house prices in the Welsh heartlands, first language speakers would be able to stay there and keep Welsh as the language of community and businesses. If child learners had opportunities to do more with the language after leaving school, they wouldn't all disappear off the radar. If young people and other professionals in Wales could learn Welsh as an actual part of their work, with clear benefit, the drop-off rate wouldn't be so great. I guess it all requires a lot more money, planning and will for the language to survive than is currently available.