NanKate, you could try downloading another spell checker. Having used quite a number over a period of time I would recommend "Grammerly". The app is widely used on Google Android devices, but I would not know if it is available for Apple devices as I never use any Apple products.
I did read that Nueance who have produced a number of spelling checkers, text to speech apps and voice recognition software for Apple have now withdrawn support for their products on that platform. It could be that your current app is caught up in the situation between Nueance and Apple if it was developed by the former.
Hope the above helps.