A lack of the appropriate munitions - launching sorties with one 500lb'er to keep sortie counts up,
I can attest to (or confim) that comment from conversations (frequently drunken) at the time with the people who were doing it.
Some might recall the US (at huge expense) buying back a large number of iron bombs they'd sold some years earlier to a West German fertilizer company as war surplus,
(circa 1965-66? - not certain of the date).
However, in the meantime, inter-service rivalry saw someone vaguely naval in the Pentagon, in an imbecilic effort not to allow the USN's sortie rate to fall behind the USAF's, launching multiple flights of A4s into the most concentrated and sophisticated AAA environment known to man (to that time)
with concrete blocks on one wing to balance the single 500 lb bomb each aircraft carried(!)
Read Jack Broughton's very readable 'Thud Ridge' for a USAF perspective on the same subject.
(The description of Broughton's wingman's F105D flaming out over Laos in sight of the tanker and his subsequent 'dead stick' approach to the tanker is worth buying the book for alone*.) He mentions exiting through Haiphong Harbour and getting the finger from the crews of merchantmen - merchantment from countries, some of them Western (read France and the UK), that the US (in his words), had saved from the Nazis in WW2.
The micro-management of the war, particularly the restrictions applied to targets in the North, was little short of criminal. Again quoting Broughton
(probably inaccurately - it's a long tme since I read the book), he says that the USAF planners cited 100+ targets in the North that had to be hit in the first days of any bombing campaign. Quite late in the war, 109 of this list of targets had yet to be touched.
What quite a few people might not know is that the Ho Chi Minh Trail, whilst certainly important to the North Vietnames war effort, was as much a propaganda myth as a fact. For much of the war, the vast majority of war material that reached the Viet Cong and the NVA in South Vietnam had a very short journey overland after being being unloaded from (neutral) ships in (Siahnouk's neutral) Cambodia, where politics dictated they were untouchable.
(Standing by to be reminded of the rephensible American bombing of the border regions of neutral Cambodia.)
(* Reading the wingman's version of events is - understandably, I think - a little unkind to Broughton as the element leader who got him into that particular pickle!!! See '100 Missions North/a Fighter Pilot's Story of the Vietnam War' (ISBN: 0028810120) Kenneth H. Bell )