That's really disappointing, sorry you have to be dealing with that. Your bridesmaids are the very ones you should be able to depend on for the hen party. Did she say why she is dropping out of the hen? Could it be because of cost and she might be too embarrassed/proud to say anything? Is there anything else going on in her life that might be making act this way? If she is having problems in work/relationship/family etc.. then maybe she doesnt want to be away for a weekend (if that's what you are doing) or she mightn't be in the right headspace and knows she won't be good fun if she went. Just throwing out suggestions as to why. It might be worth sitting down and talking to her,tell her you are disappointed she isn't coming and how you were looking forward to doing x,y or z with her and maybe ye might resolve it. It's very bad form for a bridesmaid not to go on the hen so hopefully there is a better explanation or else she is really showing herself up.