There has also been confusion around what is "free" as part of the scheme, and what has to be paid for.
The government-funded hours cover term-time only, and providers say the funding rates, particularly for children aged three and four, are lower than the costs.
It means many nurseries are putting up their prices. A University of Bath study tracking fees over the past 18 months found that they have risen fastest in areas with the lowest government funding, which it says could deepen regional inequalities.
"Parents are phoning up, they're looking for this thing that's been called 'free', and then they are met with additional charges, for meals or nappies," says Sarah Ronan, from the Early Education and Childcare Coalition, which represents childcare providers and charities.
"The sector has been tasked with rolling out the biggest expansion of childcare in history, and they're doing it in a really constrained financial environment."
She says without extra funding, providers may reduce the number of hours they can offer and pause their recruitment plans, further limiting the availability of places.
Joeli Brearley, founder of the Pregnant Then Screwed campaign group and the parent support programme Growth Spurt, says there is "a tussle between parents and providers" who are both struggling.
"For parents, it's really complicated, it's not really working," Ms Brearley says.
"We are hearing from parents who are moving their C-section day in order to fit in with the funding criteria, we're hearing from women who say they've gone to their midwife for a sweep to try and bring labour on faster, and people that are asking for inductions earlier just so they can fit with the funding criteria - and that is madness."
A survey by Growth Spurt and Women in Data suggests that many parents are paying extra consumable fees of £15 a day.
The government has issued guidance saying any additional costs need to be laid out clearly and are optional, but nurseries say charging for extras is the only way to make up the shortfall.