61 Chevy powerglide shift issue

Hey all you chevy guys out there...struggling to get a 61 powerglide to shift from low to high. Sometimes it’s perfect, sometimes won’t shift at all, but usually after it shifts the first time it will continue and work good. Just drove the car on a 5 hour trip and worked great so I feel the bands and drums are good. Fluid level is perfect. Also every now and then when under heavy load from standstill it will feel like it slips just a bit. Band adjustment? Governor? Any help is appreciated!!