We hope to fly from Siem Reap to HCM City next February. I can't seem to find any airlines that fly between these two cities. Any ideas ?

Viet Nam Air used to fly this route. Check their website, I expect they still do.

Current schedule with Vietnam Airlines Siem Reap to HCMC shows 4 flights a day,
1330-1430
1720-1840
1820-1920
2025-2125

http://www.vietnamairlines.com/

Flights for February next year though probably not showing / planned yet.

I wouldn't trust the VN Air website yet. It's not fully functional yet. I tried looking for flights from SGN-BKK and it said they didn't fly to BKK that day. They fly there everyday at 10:30am, so the website is not accurate.

There are 5-6 flights daily between Siem Reap and HCMC operated by Royal Khmer Airlines and Vietnam Air. Here's an example of flights for July 1, 2007.

Vietnam Airlines Direct Flight
08:25 (1 Jul) Ho Chi Minh City Flight number - VN 0809
09:45 (1 Jul) Siem Reap Aircraft type - AT7
1hr 20mins Total Travel Time

Vietnam Airlines Direct Flight
11:40 (1 Jul) Ho Chi Minh City Flight number - VN 0827
12:40 (1 Jul) Siem Reap Aircraft type - 320
1hr Total Travel Time

Vietnam Airlines Direct Flight
15:30 (1 Jul) Ho Chi Minh City Flight number - VN 0823
16:50 (1 Jul) Siem Reap Aircraft type - AT7
1hr 20mins Total Travel Time

Vietnam Airlines Direct Flight
16:30 (1 Jul) Ho Chi Minh City Flight number - VN 0829
17:30 (1 Jul) Siem Reap Aircraft type - 321
1hr Total Travel Time

Vietnam Airlines Direct Flight
18:35 (1 Jul) Ho Chi Minh City Flight number - VN 0849
19:35 (1 Jul) Siem Reap Aircraft type - 321
1hr Total Travel Time

Royal Khmer Airlines Direct Flight
19:35 (1 Jul) Ho Chi Minh City Flight number - RK 684
21:30 (1 Jul) Siem Reap Aircraft type - 732

Thanks for all your help. I tried Vietnam Air and their website showe no flights. At least now I know there are regular flights so we can do it.