Emily(9) has had several bouts with DIOS over the last year and one in particular was awful and dragged on for a few weeks; we tried several things to get her cleaned out (Miralax in increasingly huge doses- 8 caps in 32oz of gatorade, milk of magnesia, mag citrate, etc), repeated xrays showed it getting worse instead of better and eventually she needed Golytely to get it resolved. Even then it took 9 hours and 2.4 liters for results! Yes, it's miserable and frustrating and I can't even tell you how much I hated CF at that moment for my daughter, watching all she had to go through. I am so sorry you are going through this.
She is on Creon and takes a maintainance dose of Miralax daily now. We have to keep close tabs on poop daily now and at the very first signs of obstruction (the scant loose stool) the Miralax goes up. During illness, the Miralax goes up, since she's especially prone to obstruction when she's sick- this is documented if you read about it, and it's certainly true in her case.
She DOES now get 'that feeling' sometimes and has started telling me 'I can't poop right' and asks for an extra 'Miralax feed', in which case I give her an extra cap in a full bag of gatorade via gtube (I love the gtube for times like this!) I have no idea what in particular that feeling is, she can't explain it- I am just glad she can recognize it and knows to ask for extra. Sometimes she'll still have what looks like normal poop in the toilet when she tells me that- but other times she does not, so if she has 'that feeling', I am certainly going to act on it.
So far, being extremely vigilant about the daily stool pattern has helped and we've escaped a major bout for a few months now. If/when she has another, the dr wants to try Lactulose as well as mucomyst (taken orally- or in her case, she'd be fortunate enough to be able to escape the taste and get it via gtube.)
I hope this doesn't become a repeated issue for you.