Have you contacted your pump team to ask for advice? It takes time to adjust from mdi to a pump, and it could be that your basal rate is wrong, or carb ratio has changed etc.
If you're using Teflon cannulas, have you tried the steel ones?
You could also ring the tandem helpline, and report the issues you're having to them. I know they're trying to bring out a new cannula, and are logging all calls from anyone that's struggling, so the more people who ring the better.
When you say the level in your cartridge is going down, are you using Control IQ, as the pump will automatically correct when you're high.