You are soldering to the tinned areas of the cells and not the silicon portions, correct?
The one panel I created by hand years and years ago I remember as being initially a challenge. I had built a number of effort intensive heathkits prior to that with lots of soldering.
Soldering PV cells takes a bit more effort, or so it seemed to me. Flux on the traces may help, pre-tin the wire or strips you are using for interconnects may also improve the success.
Expect to break a few learning unless you are extremely lucky.