You don't mention exactly how much your plane weights. You say it is"rather heavy". Is is above the mfg given weight or not ?

Also, GP suggests a .10-.25 engine. Your .15 is in the lower part of this range. It is possible your SlowPoke will fly with this engine, given it is handstarted.

When taking of of grass, no matter how short it is, always produces extra drag (the small wheels have trouble rolling through the grass) which may not be overcome with a small engine.

Try handlaunching it (if the weight is like the mfg recommendation!) and see how well is flies.

A Friend of mine had the same problem with his .25 trainer plane. If handstarted, is would fly rather fine, with enough power to pulls loops and so, but there was no way he could get it off from the ground. Just to much drag on those wheels !!!