It happens, we’ve all heard the stories. Bilingualism comes easily to some, and in some situations, and for other families and situations, it’s a struggle. So if you are a few years down your bilingualism road and you feel like you are not getting the “results” you expected with your child, what can you do? In many cases, professionals will tell you to just drop the “extra” languages and be happy with having a monolingual child. I think that is very rarely good advice (see this post: Dropping a language?). If you are dedicated to your languages (and in most cases you should be!) what are your options if things just aren’t working?
Basically, you can look for help in one of three areas.
Firstly, if your child is showing delays in all languages, it would be a good idea to have an evaluation with a Speech Language Therapist (SLP). Ideally, a child should be evaluated in both/all languages, in order to get a clear picture of language development in each. If that is not possible, it’s very important to work with an SLP who understands bilingual development and who will work with the parents to understand the child’s global language development.
A second option is to work with a professional in bilingual development. This is useful if you feel that your child is really only using one language, and isn’t developing the second/other language alongside. A professional with training in bilingual development may be able to help you identify ways and means to adjust your family language plan in order to better work towards your goals. This can be as simple as identifying input needs, or helping to plan for a structured enhanced input for a lesser used language. In all cases, they can help parents understand what elements are within their control in terms of maximizing their children’s potential.
Finally, in more complex cases, an educational psychologist can help parents identify learning issues that may be impacting language use or development. Many children with Autism Spectrum Disorders struggle with different aspects of language development, and other educational challenges also impact language or involve language use. While children who have special educational needs may struggle more, they can certainly, with the help of their parents and a dedicated and knowledgeable professional, successfully learn to use more than one language.
In all cases, I advise parents to reach out for help as soon as they start to have concerns. The right professional can either put your mind at ease, or get your child immediate and accurate help if needed. In either case, parents are saved the stress of worrying “what if”, and of trying to figure things out themselves.
on raising bilingual children 
I agree with you that seeking help is important as soon as parents think that bilingualism (or multilingualism) is not working like they expect. But how can they recognize if it’s only a matter of time – every child has a different way to cope with languages etc. – or if there is really something that needs professional help. I know that the range of “signs” to seek for help is huge, but maybe there are a few more general ones who can help parents? In your experience, is there a time frame within which for example a delay in speaking the languages or becoming is considered “normal” for multilinguals? Or is it also possible that this would be “too late”?