OK so some but not all texts fail to send?
Any error message?
When a message fails to send to a particular number can you ever send a message to that number or does it always fail?
Would they happen to be long messages?
If they are then your messaging app could be converting them to MMS messages which will only send if you have a data allowance, your internet settings configured correctly and cash credit on your account to pay for them (30c each).
Some messaging apps allow you to override this default behaviour and always send messages as SMS text messages rather than converting "long" messages to MMS messages.
But some apps do not in which case it might be necessary to download/install a different (free or commercial) messaging app that does allow for this.
If that's not the issue then could it be that you are trying to text to a 5XXXX premium SMS number (e.g. radio competitions etc.) which are not supported by 48?
Hope this helps.