Did they not fully realise the potential benefits at the time/thought they wouldn't work well?
Turbofans and turboprops were already studied in Britain and Germany during WW II, but at that time were technically not yet possible. Their benefit was indeed clear, even though the lower noise of turbofans was less of a consideration than it is now.
or was it just very difficult to build a jet with useful excess turbine power (to drive the fan) at the time?
Yes, that was the reason for the first jets. The pressure ratios of early jet engines did not allow to add a fan, and also the added complexity of a second spool was seen as not economical.
However, the next generation of jets could have supported a fan but didn't because they had been developed for fighters. Here the goal was to fly as fast as possible, and that needs low frontal areas and high exit speeds. Both would have suffered if a fan had been added. Civilian use of jet engines was initially restricted to the Comet, which buried its engines in the wings and could not accommodate a fan. Same for the V-bombers. Only Boeing's bombers could have benefitted from turbofans, but aerial refuelling made their low engine efficiency less obvious. Note that the Pratt&Whitney J-57, which was one of the first two-spool engines, was a pure turbojet for the first eight years of its life.
Only when the market volume of jet transport aircraft grew to a sufficient size in the late Fifties did the engine companies start to add fan stages to their existing engines. The first was the RR Conway, which had been developed for the V-bombers and had a bypass ratio of only 0.25 due to the diameter restrictions of the existing designs. The J-57 turbojet was developed further into the JT-3D turbofan in 1958 when the range of the first, turbojet-powered, Boeing 707s and DC-8s turned out to be marginal for transatlantic flights.
Yet early jetliners first used turbojets then only very low-bypass turbofans, despite the existence of turboprops
There were civilian turboprops for short range flights, but the speed advantage of jets meant that all new types developed for more than regional flying had to be jets in order to be competitive. Note that the latecomer in the trio of Boeing 707, Douglas DC-8 and Convair 880 used a smaller fuselage diameter and four Starfighter engines in order to gain a speed advantage over the other two.