Do you like hanging out with them because their your friends or because their girls? I mean it doesnīt have to contradict, but if you mostly see them as girls you might become interested in as girlfriends than you will probably have to approach it differently than when you just consider them to be friends.
Iīm shy, too, and Iīve also had my problems in maintaining a conversation. What Orchid said is true though, people like talking about themselves, ask questions and they will talk (unless they are bloody bastards who just donīt like talking to you....

Iīm just being mean

).
--Donīt ask questions that can be answered with yes or no, ask questions with open endings.
--Learn to have an opinion, thereīs nothing more boring than talking with someone who doesnīt have any opinion at all. Even if you donīt disagree itīs better, at least you have something to discuss.
--I know other people like to share their ideas, thoughts and are able to bond only via email, messenger, etc., but I think there are also other people who need to have some common background or memories to feel relaxed to talk with a person, you do need to spend time with them, not necessarily parties, but maybe do an excursion, share a hobby with your friends, help them with something, create a mutual bond of friendship and then you will feel less tense when youīre with them, if you have their confidence as friend, talking about many things, also problems, will become easier.
--Real life conversation can be different than talking on the internet, some people are just not very good in conversing on the internet. Iīm also not sure what the intention of the girls is when they chat. Often people want to flirt and if you are looking for a deeper conversation thatīs just not what is wanted or expected.
--As a shy person I know that we often expect others to make the first move when it comes to talking. Thatīs a bad idea.... Start to show some interest, but donīt be too compliant or too eager (if you are interested in more than friendship, thatīs an important advice

)
Eventually , there might be the possibility that you guys just donīt have anything in common and thatīs why you donīt hit it off so well.
Good luck