Not receiving all forwarded e-mails from another account in Hotmail

I have set up my e-mail account with my local internet service provider to forward all incoming mails to my Hotmail (now Outlook.com) account.  This normally works fine but I sometimes notice that certain mails are not forwarded; I only receive them in my (hard drive) MS Outlook, not in Hotmail.   I've searched the junk and other folders for it but they are just not there.  I tried to figure out which e-mails specifically are not forwarded; it are mails sent directly to me (e.g. list serve mails all arrive normally in Hotmail as well as newsletters), but I cannot extract a specific pattern to be able to predict or determine exactly which are and which are not forwarded.  Any similar issues; answers why this may be so?  Thanks!