Bitty Baby's Head keeps falling off-HELP

My daughter's beloved bitty baby's head fell off today and I keep putting it on, tying the strings tight, and an hour or so later, it falls off again. The doll is in perfect condition and my daughters are very motherly with their dolls. She has just been pushing here around in her bitty stroller and putting her to bed in the yellow bunkbed I got of ebay in the mail a couple of days ago (I love it!). Any ideas of how to keep it from falling off???

I posted about this here, maybe it will help: http://agplaythings.proboards105.com...ead=1156535500

Thanks, that helps, I have the plastic ties right here and will give it a try!!!