I don't know what texting program you use but I use handcent and one of the options is to split the message if it exceeds 160 characters. This may be an option to alleviate the problem you are having sending long messages.
In handcent there is also the option to check for characters that may not allow for the sms/mms to be received by the recipients carrier. this could also be preventing your friend from getting your messages.
Their phones may not be able to handle enhanced text messages.
