Some times in the journal voucher number skips the series and leaves few numbers in between.It is quite dangerous in the cash receipt and it has audit implications too.
Setting the number sequence to Continuos should do the job.
Maybe you had some modifications done (creating journal entries from code, for example). In this case you should check whether you are using the NumberSeq right.
I remember that there were some bugs on numbersequence in Ax SP3 for which MS subsequently released a hotfix. In particular the fix was for - there were times when Ax would pick up a number that was already used and then subsequently throw “number already used” error message.
If you ever had this problem perhaps you can give it a go downloading the fix.