Isn’t it the job of “Management” be it Agency or Client Yard to make sure there are always “more drivers than jobs” and “Enough drivers to fill all jobs, in case some say ‘no’”?
People have never been motivated on the whole to leave full time to join an agency as such… Unless one can get by with less than 40 hours per week of work for possibly prolonged period.
I, for example - need enough work to get my stamp paid each week, with the second to late week of the month being the only time I need “full timer’s hours” that week, for instance.
Never had any trouble getting that, - so far.
It is the nature of irregular weekly hours that when it is all paid at a flat uniform rate - one takes home progressively less cash as one advances beyond tiers of hours each week…
Eg. 1-2 shifts - take home nearly ALL of the pay, as there’s little or no tax, it all being within the tax allowance.
3-4 shifts (which I favour) - take home a normal wage, pay a reasonable amount of tax
5-6 shifts - stuck on same flat rate, but now paying full tax on ALL earnings, having moved way beyond allowances. Rule of “reducing returns” applies.
I am thus relucant to work shift (5) each week if I then won’t be getting shift (6) to pay the bloody tax on top!
I cannot work six shifts per week all the time, neither.
What use is a “Any five from seven” contract for ME then? 
For me it is 4 or 6 - with five shifts in a week happening as little as possible!
…If the start of the week is 00:01 Sunday morning, then I won’t be working a 5th shift that week for any reason, if my first shift then ends up being "Monday", so no “Monday-Friday” for me, and I’ve fortunately not been compelled to work 5 shifts that doesn’t include a sunday since 2018, when I left full time, and chose to go back on agency, rather than pick up one of these “Any five from seven” jobs that I just don’t want. 
It is possible that over the coming weeks, a glut of sideways-moved full timers will deplete the agency of spare shifts for me to work.
…As long as I get at least that “Retainer” shift of Sunday then - I will be able to get by, even if the work is “short hours” for a prolonged period.
If the agency stops using me for any entire week at a time - I will be relucantly forced to move on that very same week.
Full time jobs @ supermarkets - appear to be becoming available again BUT it is still the “Any five from seven” family-unfriendly hours rubbish that I’ve tried so hard to avoid to date…
I’d already jump at the chance to do Four-on-four-off or even “Any four nights from seven” - but they just are not out there, alas… 