I can't add schedules to my B737 aircraft in KSMF base. I tried and an error message comes up that says "ERROR-Cannot assign this route for this aircraft. The route would overlap another flight or maintenance check already in the schedule." However, I can move existing routes between aircraft. The routes numbers are BD1079/1080 KSMF-KMKE-KSMF and route numbers BD1081/1082 KSMF-KJAX-KSMF. I edited route BD1079/1080 to reflect an ERJ-145 aircraft and I was able to add the route. When I switched the route back to a B737-300 aircraft I got the same error message.