Is it proper to use this idiom here: "...I'm sure there is more to it than that."?
The sentence: I doubt it is a coincidence. I'm sure there is more to it than that.

Is my sentence correct and natural with the idiom: "there is more to it than that"?
If not, can you suggest a sentence with the same meaning?

Thank youI want it to mean: I don't think it is a pure coincidence. / I'm not sure it is a coincidence.

Question

Is it proper to use this idiom here: "...I'm sure there is more to it than that."?
The sentence: I doubt it is a coincidence. I'm sure there is more to it than that.

Is my sentence correct and natural with the idiom: "there is more to it than that"?
If not, can you suggest a sentence with the same meaning?

Thank youI want it to mean: I don't think it is a pure coincidence. / I'm not sure it is a coincidence.

Answer

"I doubt it is a coincidence." and "I doubt it is a pure coincidence."  are both the same in meaning. However, "I doubt it is a pure coincidence." could be that it was somewhat a coincidence, just not a 100%.

Answer

Not knowing the complete context, it seems perfect. Great job.