From the HH terms and conditions:
- Diamond Elite Members will receive the following during stays as a registered Diamond Elite Member: At Waldorf Astoria® Hotels & Resorts, LXR Hotels & Resorts, Conrad® Hotels & Resorts, Canopy™ by Hilton, Hilton Hotels & Resorts, Curio - A Collection by Hilton and DoubleTree by Hilton™ and Tapestry Collection by Hilton™ properties, Diamond Hilton Honors Members may receive upgrades to preferred rooms, based on availability at the time of arrival. Upgrades for Diamond Hilton Honors Members may include upgrades up to “junior”, “standard” or “one-bedroom” suites. Upgrades exclude executive suites, villas and specialty accommodations/floors/towers (which may include, but not limited to, "Vista," "Villa," or “Club” accommodation types), subject to the discretion of the hotel. Group reservations and certain rates are not eligible for suite accommodations and may not be eligible for complimentary upgrades. All upgrades are granted on a space-available basis for the entire stay, as determined at the time of arrival. Preferred rooms are identified by each individual property and may vary within each brand. Upgrades will be given only for one room for the member, regardless of additional rooms the member may have purchased at or after the time of booking. The following brands do not offer complimentary upgrades: Embassy Suites™, Hilton Garden Inn®, Hampton by Hilton™, Tru by Hilton™, Homewood Suites by Hilton®, Home2 Suites by Hilton®, and Hilton Grand Vacations®.
Preferred rooms may also include those not on the Executive Floor but conferring Executive Lounge access, the next-best available room types, rooms with desirable views or amenities or other rooms identified as “preferred” by the hotel and may vary within each brand.
The t and c's provide a lot of wiggle room to individual properties, use "may" not "shall", and with the preferred room language can include rooms that do not necessarily even cost more but have some other quality defined by the property as being better. I usually see more upgrades when I check in later in the day and for single night or other short stays.
If you are upset or unhappy, write a letter to Hilton, provide negative feedback on the survey, complain to the GM either before you leave or after. Me, I let it go and enjoy the room I booked, and if it really bothers me enough I book elsewhere in the future (it usually doesn't -- my choice to not return to a property is usually based upon other factors). If I like the hotel I usually don't book elsewhere and most places, as a return guest, I usually see more upgrades.
See these threads:
Check in agent lies about upgrade availability - what do you do?
Why a Diamond is Not Upgraded Despite Availability
When did Hilton properties start denying upgrades in advance?!
Diamond yet never upgraded!
Never been upgraded as a diamond
A vent - why do Front Desk Associates Say "sold out?"
How do you handle denied upgrade when there are rooms available?