Never have this problem. See this.
http://pantip.com/topic/31484570
http://pantip.com/topic/31447838/comment2
If you really understand the English passage 100%, there is practically no chance that you cannot render the equivalent meaning in Thai.
It is the degree of your language competency in both languages. This is simple. If you ever take a standardized tests such as Toefl, CU-TEP, TU-GET and you can earn the score at the range of 95-100%, I am sure that you will never have the problem of translating in Thai equivalent.
Another factor is the knowledge field. If you read widely and extensively, your knowledge field will be wider as well, and thus it is easy to comprehend the passage and then it is real easy to translate into Thai.
This takes years or even decades. There is simply no short cut to master the English language.