Hi,
You may do it as following.
1. Check from SRX - if you are able ping - 10.1.10.20
2. Check from SRX - if you are able to ping - 10.1.20.x/24 (host ip)
3. From host ip (10.1.20.x) - are you able to ping default gateway - 10.1.20.1?
4. From 10.1.10.20 - are you able to ping default gateway - 10.1.10.1?
5. If answer to all the above question is yes - then proceed with next step. Otherwise, you may want to check cabling and/or allowing ping in the host or servers. Also, hoping that Wireless hosts are residing in segment 10.1.20.x/24 only.
6. Try pinging from WLAN zone to Internal server - at the same time, check the flow session - run show security flow session source-prefix <10.1.20.x> destination-prefix 10.1.10.20
7. check if you are able to view forward and reverse packets
(a). - if forward and reverse packets are increasing correctly - then flow is formed correct. Application should work in such scenario.
(b). if forward packets are increasing and reverse packets are zero in the flow - try deactivating nat-ruleset wlan-src-nat
(c). perform the ping or run the applicaiton again.
9. if nothing works, then try putting up the permit policy from 10.1.10.0/24 to 10.1.20.0/24 segment and check.
HTH...