This is for a 2003 Ducato 2.8:
The gearchange works with two cables; one for fore/aft and the other left/right. There is a nylon block in the dashboard fitting under the gaiter to which the top ends of the cables are attached, and the other ends go to separate attachment brackets on the gearbox case. The nylon block can distort under force, which doesn't help.
You may just have a failing cable (possibly just needs lubrication), or one of the brackets on the gearbox could be bunged up with crap (I had this initially, and it may be the root cause), making 5th hard to get. My problem (which started like this) gradually got worse so that having got 5th, getting 5th again was easy, but 1st & Reverse then became really difficult. What happens is that a selector inside the gearbox getts bent slightly each time this happens, as as this is a light casting it eventually fractures, requiring replacement, which isn't particularly cheap, but cheaper at a transmission specialist rather than a normal garage (who would probably farm it out anyway).
I ended up with a rebuilt gearbox and two new driveshafts...
Sorry if this isn't great news, but it's better to know the potential worst before it happens!