Hi Again, Aldijkhuizen,
It seems strange to me that Waters would publish a completely poor method or two that would not "work out of the box," so-to-speak.
There's no doubt that you want to have basic conditions for these separations and with this phase (amino).
Are you certain that the strong eluent and weak eluent in your gradient program are not inadvertently reversed on the instrument? Remember for HILIC, water is the strong eluent and the organic component (ACN) is the weak one. This could cause tailing, I think.
Otherwise, the variables you have to work with are temperature and salt type and/or concentration to eliminate tailing.
Remember also that HILIC takes a Long Time (twice as long as RP) to equilibrate, that may also make a difference.
Best Wishes, again. I'll keep thinking about the trouble(s) to see if I've missed anything. The next step would be to change phases I'd think.
Mac-mod has a nice guide for that:
http://mac-mod.com/pdf/product-bulletin ... 0Guide.pdf