BBC Wales has heard claims that one subject, launched 15 years ago, still has no teaching resource in Welsh.
Shadow Education Secretary Llyr Gruffydd said it was "unfair" to rely on teachers to translate material.
The Welsh Government said it was committed to making the "right resources" available.
Chris Evans, head of psychology at Ysgol Morgan Llwyd in Wrexham, told the Newyddion 9 programme that he spent his summer holidays in 2015 translating a textbook for his students.
He said he complained at the time but claimed nothing had changed, with the same problem arising with the second year of the course.
"The English language textbook was published in June but the Welsh version will not be out until at least next summer," he said.
"Translating the material has put added pressure on me over the last two years and put Welsh language students at a disadvantage."
Mr Gruffydd said: "It isn't right that a course is introduced until all the necessary resources are in place.
"I understand the need to ensure that English language textbooks are available and that there is a commissioning process to have them translated.
"But it would be a simple matter to delay for a year until the Welsh language provision is available.
"It is unfair that teachers have to translate them and it devalues the worth of the course in the eyes of the students."
Responses to a survey by teaching union UCAC suggest several GCSE and A Level subject are without adequate Welsh language material.
Some teachers have claimed the health and social care GCSE course has had no Welsh language text book since it was launched 15 years ago.
Others say maths textbooks become available when the course is nearly completed.
A Welsh Government spokesperson said: "We are committed to getting the right resources available to support the teaching and learning through the medium of Welsh.
"The minister for Welsh language has received a letter regarding this matter and we are looking into it."
The launch of new GCSE and A Level subjects should be delayed until Welsh language textbooks are available, Plaid Cymru has said.